Is google limiting keywords per site?

[teseo]

I have been doing a check about how are my indexes going in one of my sites and i have noticed that google has limited the indexes per keyword.

E.g:

If i type site:mysite.com keyword i get the result of 6680 in total
if i type site:mysite.com keyword2 i get the result of 6680 and i got that amount of result for a couple of keywords

but:
if i type site:mysite.com keyword3 i get the result of 5090.

So depending of the keyword, i get different number of results but no more than 6680 indexes.

Does anyone know anything about keyword per site?