IEEE and ACM gives very different results Me and my colleague are conducting a literature search that will be used to argue about an experiment. We've just started our SLR and noticed that ACM and IEEE gives very different results. 

When searching for "java AND jax+rs", ACM gives 26 hits while IEEE gives 18577 hits when searching for "java AND jax+rs". The term "java AND jersey" gives 25 hits at ACM and 701 hits at IEEE. In other searches, such as "auth AND oauth*", ACM gives 62 hits while IEEE gives 65 hits. 

We are using the same filters in both searches (limit to journals and proceedings, all years, metadata only). When searching at ACM we limit the publisher to ACM only, and at IEEE we limit publisher to IEEE.

Are these types of results usual? I understand that they are different publishers and thus publishes different papers but these results seem off.
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.