You've overlooked nothing other than the fact that the HWID of the unit remains as 0827, therefore WU actually sees it as a nuvi 2x5W series because it reads the hwid regardless of which model number it displays. If you think about it, your 2009 [so called] '250W' has the same model no. as earlier [real] 250W's but has a different HWID. The fw required is based on the HWID [normally], so WU will supply the correct fw for it accordingly. If you did a full conversion to 465T with hwid 0943 then you'd get the 465T fw.

If there are any further fw updates for 465T from Garmin [possible], just use RGN_Tool to combine the 0827 boot.bin with the new 0943 fw_all.bin and flash in preboot.