The definition of SRM scale is based on the absorption of light at a single wavelength, so it's only measuring one aspect of color.
The way the SRM views color is similar to how things look when you put them behind a yellow filter.
Beer color is of course more than one-dimensional - reds, oranges, even some green, but these are not taken into account with the SRM which is focused on yellow.
Wikipedia goes into more detail:
Determination of the SRM value involves measuring the attenuation of
light of a particular wavelength (430 nm) in passing through 1 cm of
the beer, expressing the attenuation as an absorption and scaling the
absorption by a constant (12.7 for SRM; 25 for EBC). The SRM (or EBC)
number represents a single point in the absorption spectrum of beer.
As such it cannot convey full color information which would require 81
Given that 430nm is in the blue region, measuring the absorption of that tells us how much blue light the substance absorbs. But it doesn't say what other light is reflected/absorbed. So, a completely red beer and a completely green beer would look quite similar on the SRM scale, since these colors absorb pretty much the same amount of blue. What makes them appear different is that they reflect different amounts of other colors, but this is not measured in the SRM.
If some other scheme was going to try to represent color more accurately, you'd need to start writing multidimensional numbers like (15,28,90) to represent multiple absorption frequencies - and compared to a single SRM number these would be much harder to work with: is that beer darker or lighter than (80,8,25)? Since the eye perceives the different frequencies at different levels of intensity, intensity is non-linear.
So, while a single SRM figure can't capture the full spectrum of beer color, the trade-off comes at much greater ease of use.
You won't get a red color with chocolate malt - for that, try some of the medium crystals (e.g. CaraRed) or even roasted barley, which gives ruby highlights.