A single Q/V score combination can produce different scores out of 800, so even if you had a perfectly reliable score chart (and knowing how prep companies make these things, I doubt one exists), there would always be a chance it would be off by 10 points sometimes. The thing is, if you get, say, Q46/V35, those scores are rounded off -- it might be more like a Q46.4/V35.4, or more like a Q45.6/V34.6. The score out of 800 is determined before the rounding is done, so the Q46.4 combination might produce a higher total score than the Q45.6 one. I don't think I've ever seen a difference, when two people have the same Q/V split, of more than 10 points in their scores out of 800, but 10 point differences are common.
That said, when I quickly google "Q46 V35 GMAT", it appears that score combination produces a 660 or a 670, but more often a 660, so I think the score charts you're consulting are a bit off.