Afaik, R27 is a middle range missle

Missle in itself is just a "metal, fuel, TNT". Flyable range of missles is just a figures on paper.
Weapon, especially modern A2A and A2G weapon - is a complex of various parts, including carrier, target selection system (s.a. radar, eos)... So, Su-27 (carrier) with it`s radar N-001 can use R-27RE at ranges of 20-25 km, inspite of ~40 km R-27RE flyable range. More: lock range of any radar depends of various factors, s.a. weather, target dimensions, altitude, "stealth cover"...

In addition, radar system of F-15 in "LoMac version" is a bit better, than radar of Su-27 ("LoMac version" too ), and Eagle can lock targets and launch it`s AIM-120 missles at greater distances than Su-27.

So, this is LoMac reality: Aim120 gets first shot over R27RE/TE