I see a similar effect too. I usually do just lifting but I recently started doing a bit of cardio (rowing and that resistance treadmill thing) and it improved my lifting even at 5 reps. Granted my cardio was pretty bad but I felt more steady and felt like I could do more volume.
Completely anecdotal but I think cardio of High-Intensity Interval Training (HIIT) type transfers over the best to weight lifting. I am going to guess it has something to do with the intense/explosive intervals in cardio making one more used to similar explosiveness on weights.
