IEEE and ACM gives very different results
Big data systems like search engines are really complicated algorithmic engines that put everything through a giant sausage-making apparatus to give you your results.  Even if the two databases are running the exact same engine on their back-end, there is no reason to expect that they have configured it the same way or that the results are necessarily stable.  This is especially true for recent "smart" search engines which try to figure out what you might have wanted, in addition to what you actually entered, and which may broaden your search for you whether you ask them to or not.

In short: every big database search is likely to have its own quirks and ways of approaching queries, and it is reasonable to expect that they may be interpreting what you are asking in radically different ways.