Understand that part of the difference is due to age and lower production numbers (roughly 770 275 GTBs vs. 1300 365 GTB/4 Daytonas) but why are 275s values 3-4x higher than Daytonas?
They look "sexier" to most, nice, round and "aggressiv". As you say: Add up the almost double production size of the Daytona and you got the mixture for higher prices. Personally I think that the price is rather 2-3 times the Daytona value (except for the real rare Alloy 275 GTB's as seen this weekend's US auctions)
Although the styling and build numbers contribute, I believe it has even more to do with the racing history. The 275's were used, with hardly any modifications, at LeMans, Tour Auto and many other road races that were dying out as the Daytona came to life. Although some Daytonas were raced, by that time the cars became much more specialised and a race Daytona is quite a different beast than the 275 Alloys.
I believe the fact that the race 275's are so much like the road versions gives them an even more romantic and desirable vibe than the Daytona.