I just thought of something which might have to do with it!
Could the amount of plates matter? Let me explain:
I have a very cheap bar (~15 pounds but I round down to 10). When I got 295*4, on each side I had the following plates: 50, 25,25, 15, 15,10, 2.5. But when I went for the 1RM all I had on each side was: 50, 50, 25, 25, 10
??
-Zulu