Speaker placement and dB differences

So I’m pretty anal about speaker placement. I use tape measures, along with laser lights and levels to assure proper and equal placement from walls. I was using my new Omni mic and testing software to run some white noise frequency graphs vs db range to see where I stand.  After overlaying the R/L graphs I noticed that there is about a -1db difference in the right channel generally across the entire frequency range and about -5 dB at about 250 hz. 
Also my room is 12’ x 13’. 

I remeasured my speaker distance from the seating position wall and I noticed the left speaker was 96” from the front the chair and the right speaker was 98.5” from the chair. By my calcs that would only account for a .22 dB difference. Not enough to account for the 1 dB drop in the right speaker and certainly not the -5 dB at 250 hz.
Question is, is a 1 dB difference between two speakers normal. My speakers are Thiel CS 2.4’s and I’m using an older ARC tube amp - which can certainly account for 1db.

the bigger questIon is can a 2.5” difference in room placement cause a 5 dB difference at 250 hz?