2010 MBP anomaly

The 13" models feature the older Core 2 CPU's, while the 15" and 17" featured the newer first gen (at the time) Core Westmere die (32nm die shrink of Nehalem, the first gen/release of the Core i series).

With the Core i Nehalem/Westmere CPU's, Intel integrated the Memory Controller into the CPU itself, so they were working stuff out still until the second gen Core i CPU's (Sandy Bridge/Ivy Bridge) where they improved on the design and supported a max of 16GB for mobile CPU's again.

Intel's "Tick Tock" method is actually pretty interesting once you understand how it works. It's a good read:
https://en.wikipedia.org/wiki/Intel_Tick-Tock

Hope that made sense. :\