I find the "tie" thing kinda silly.
Sure, precise numbers "exist," but nobody is in a position to know them. Yes, NPD's calculations have some sort of inherent margin of error, though I doubt that there's any precision to calculating that, either (and it doesn't seem like anyone here knows exactly what their stated margin of error is, anyways, if they even release such a thing).
Regardless, NPD releases its numbers according to whatever metric they have, and one console having a higher number than another according to that metric is "winning" in this context, for whatever that's worth, even if it was by one unit.
In other words, NPD doesn't release a "range" of numbers--their baseline multiplied by their margin of error, up and down--but if they did, the PS3 range would be better than the 360 range for December. Since that's as accurate as anyone seemingly can get, that's enough to have "won" December. It's not a tie--a tie would be having the same number as released by NPD, though again the unknowable "reality" of sales for either console would exist along some range. Instead, it's a "victory."
Whether that "victory" means anything is another discussion.