Yes - if you use « a » it means « a small amount more than ». If you omit the « a » it means « not very much more than ». This second version implies that the value was expected to be higher, or at least is relatively low. It is therefore not appropriate in your sentence since there is no expectation of what the value is, and 70% is not a low value.
As is general with both 'a little' and 'a few', the version with 'a' is positive in feel, but without 'a' it is more negative. It is useful to think of 'little' and 'a little' as different quantifiers, different words.