What stats package is used by your host? Some of the stats packages supplied by web hosting companies aren't what you might call, uhm... "robust." So that's the first thing to consider.
Third, you need to make sure you're comparing apples-to-apples. Make sure you're not comparing "hits" or "page views" in one package to "visitors" in another -- those are all three very different things.
And finally, different stats packages, even if they're both very good and counting things correctly from their perspective, all use different assumptions. All stats you get are based on assumptions. There's no such thing as a 100% "accurate" stats package. So the numbers you will get from different packages will be different -- as you've seen, sometimes wildly different.
The thing to do is not to try to compare them to determine which is "right." They're both potentially right within the set of assumptions they use.
Rather, pick the one that gives you information in the form you find it easiest to use, and stick with that one. Use it to analyze relative changes -- the trends, spikes and dips in your traffic -- instead of trying to determine some absolute number of visitors on any given day.
Does that help?