I've just measured the final gravity of a Belgian beer I've been brewing and it came out 4 points below the expected according to Beersmith: 1.022 rather than 1.018. It was 14 days in the fermenter. I should say that I haven't controlled the gravity throughout the fermentation so I'm not sure it finished. Now, my issue is that Beersmith's database marks the T-58 with a 71-75 range but Fermentis sheet says 70%. According to my measurements (OG 1.078, FG 1.022) Beersmith's says that the calculated attenuation was 70%, which is spot on with the value in the Safale's sheet. Could Beersmith's values be wrong or could I be missing something here? I'm a novice so apologies if I don't make much sense.
There's many factors that effect attenuation. The %s are a ball park under normal conditions, factoring average unfermentables and ABVs. In theory all yeasts are capable of 100% attenuation of the fermentable sugars up to their ABV tolerance with a lot of nurturing.
In your case 4 points off may not be that far out of scope. If the sweetness is balanced it may be fine as it is. If not you can try a warmer secondary fermentation to finish out the few points.
The attenuation rating of a yeast is meant for comparing one yeast to another and is not necessarily an indication of the attenuation you can expect That is far more dependent on the fermentability of the wort. Using the same yeast but changing the wort, I can easily get anywhere from 65-85% attenuation. Beersmith is doing nothing more than making a guess. If it's off a few points it's no big deal.