I personally think that the reason is that cold air is denser than warm air so a few more "air molecules" get compressed when the springer is shot. Over the years I've noticed that I get a bit more velocity shooting in the cool air in the late fall than I do in the hot humid air of summer. What threw a monkey wrench into the mix was when I was lubing my R9 with molly paste and spring tar. The tar (especially "velocity tar") did/does change viscosity with temperature changes and if used in excess causes variations in piston acceleration which affects the poi. I can say the same thing about the old design thin edged HW parachute seals where the piston seal lube and the durometer of the seal itself fluctuated with temperature shifts. I have found that with my .177 R9 the new design thick edged HW piston seals perform so close to and oring sealed piston cap for all practical purposes since the sealing edge is rounded like an oring and the base is reduced to lessen "piston seal to receiver drag". This is assuming that the HW seal is a good fit to the receiver to begin with. I have a couple different new HW95 piston seals in my parts bin and found that they vary considerably in diameter. When I tuned a new HW95 for a friend I found that his piston seal was very loose in the receiver, however a new seal from my parts bin fit perfectly.
Anywhoo.......a 5fps (or even 10fps) variation in velocity is insignificant when I'm shooting!