Hi. The difference might be due to how you had the microphone oriented when you did your measurements. Dolby at least used to recommend holding the SPL meter facing the center channel at a 45 degree angle to ground (so, not pointing at the speakers or at the ceiling).
This thread contains some information on that in post six. The original link doesn't work to the Dolby website, but the document might still be out there. That's consistent with what I'd read years ago when I started doing those measurements.