I cant confirm this but I have a feeling that its to do with the total server kill count.
It might be that at the end of the round the server figures out the weapon you have scored highest with and then combines all of the other players stats for that weapon as well. So in this case you got 22 kills in the tank which means all of the other players on the server (or maybe on your team / squad) had a combined total of 5 kills.
I still dont know why they would show it like this ... maybe to illustrate your personal contribution to that particular class?
Like I said I might be completely wrong but thats the best explanation I have come up with so far. Although as with most EA games I wouldnt be suprised if it is a bug ... I was blowing up the 2nd set of Mcom stations in Arica harbour through the roof with C4 for a good 7 months before they fixed that.