The poll you cite received less than 200 responses. This is a very small sample with a large degree of error. Further you cannot reconcile the question asked in the TFL poll against what the NBC/Gallup/CNN polls asked. The media polls did not usually specify expanded checks, just BCs.
You could try another poll for your answer - something like:
NRA members ONLY: Do you support Background checks?
A) No, I do not support any background checks
B) Yes, I support current background checks for FFL transactions ONLY
C) Yes, I support expanding background checks to include private sales (excepting sales between immediate and extended family members)
Also, general poll results indicating support of BCs should not be taken to mean support of the M-T Amendment that was just voted down. That was just a poor piece of legislation, IMO.