Ashley, I understand why it's happening, I just think that not only is this majorly confusing, it's not ideal as a default.
I am beyond doubt that changing it would break a whole load of things. It's actually really nice that you can tell if an object is sitting on a platform or not for example without having to sink a detector 1 pixel down, it's just the wording is misleading.
My suggestion is to add something like "strictly overlapping", i.e. use strict inequalities instead of less than or equal. Add a strict option mitigates for the non-strict behaviour of the default overlapping condition. The boundaries only overlap because there is no convenient way of specifying (object width - smallest floating point delta), and had you been writing a game yourself in javascript you would want to use strict inequalities.
p.s. I might add, overlapping at offset is brilliant, a strict version would be sweet.