Hello Pamela,
By design obj exporters have "Optimize" settings included. What basically those do is to remove duplicate elements from the vertices, face normals, or texture coordinates (depends of your settings). That's why you see reducement of the poly count and this (export/import) acts like a cheap fix for duplicated elements that cause errors in COLLADA. I'm not a blender user but it should have some sort of statistics/checks for those kinds of errors that will help you to fix those on hand, if you like. Hope this helps a bit.
Best regards