I just brewed a clone of Belhaven Scottish Ale using the extract recipe from the book, Clone Brews. The recipe calls for both light LME and light DME. Why would it use both? It does specify brands of each to use. Might the recipe be taking into account the particular flavor of each extract from the different brands, or is there something more obvious I'm missing?
Theory the First
Since the recipe lists specific brands of extract the author could be looking for some esoteric flavor differences in the two.
Theory the Second — The Texas Two-Step Method
The liquid extract is intended to be added near the end of the boil, around the 10 or 15 minute mark. (See Brew Your Own magazine, October 2003.)
Extract brewers normally do not do a full-wort boil (all 5.5 gallons). High concentrations of extract decrease the hop utilization rate. The higher gravity wort you boil, the less hop extraction you'll get. Boiling all the extract at once in a small volume will probably throw off the hopping rate.
It has also been shown that beer tastes better when hops are boiled with some extract. Just boiling hops straight up extracts undesirable flavors.
So, you boil some extract for the full 60 and add the rest near the end just to sanitize.
Why the dry first & liquid second? Probably because LME will dissolve quicker.
To check the first theory: mix up two equal gravity worts of each extract. Taste for differences.
LME is usually cheaper, but harder to measure and store if you don't use the full package. Sometimes recipes add on DME if you aren't using even 1 pound increments. Light LME and light DME are interchangeable, as long as you take into account the higher extract per pound of DME (since it has less water it has more fermentables per pound). Also, don't switch out something like wheat LME for light DME. They aren't the same.
Another totally different, and more plausible answer, in the case of Northern Brewer's Kolsch recipe is that it's difficult to divide liquid extract to get the exact right gravity. For that recipe, 1 lb of DME early gives enough for hop extraction, and 6 lbs late gives the rest of the OG. With their raw materials, which are generally 3.15 lb and 6 lb LME jugs, and 1 lb DME bags, it's possible they want to fill in a little more gravity with the DME, without going so far as adding another full or half jug of LME. The recipe you've posted, though, does not fit with this explanation. 2.75 lbs of DME is not all that far from 3 lbs of LME, which is a pretty standard size, so still not explanation for the specific case you mention.
I doubt this is the reason, but you may want to consider the water profile of the final beer.
If you use all LME then you get the chance when using distilled water to get a beer that has the exact profile from where the extract was made. Because the minerals in their water are still in the LME. When using DME, much of that mineral profile was lost in "ringing" out the last of the water to make the powder.
So you can use distilled water for the LME portion and your "house" water with the DME addition and get something in between the two locales.
Like I said, no one at the brew shop really gave it that much thought, but I just point out the opportunity that it presents to you the brew now at home.
I realize this is five years old, but I came across it while searching for the same question, so I'll throw in my two cents in case it helps.
First thing, most LMEs are hopped and most DMEs are unhopped, so you might use both to dial in the bittering.
Second thing, it seems it's easier to find lighter DMEs than LMEs, so you might use LME for the bulk of your malt and then a light DME to round out your OG without making your final product too dark. I came across this making a recipe for a light American pilsner in Brewsmith, actually, which is what led me here.
I think using both LME and DME is all or mostly about marketing. These kits like to showcase the liquid and dry malts, different hops and specialty grains to draw you into buying more of their sponsored products. I've bought kits that required DME to be put in late.
My supplier used to sell Brewers Best kits, but discontinued them in favor of kits they put together themselves. They make different styles using different specialty grains to steep with the DME/LME that they also stock on the shelves. It's just there to lure you in toward all-grain brewing. It's their bread and butter.
I know because it happened to me. Not that it's a bad thing, I'm full on with it and having loads of fun brewing all-grain now. These kits really helped me to learn the products.