I know this is a late answer for your question, but since no one has answered it, I will go ahead.

What you are describing happens when you add a table(or query) to the relationship window that is already present in the window. So, if you add a CustomerT to the window when CustomerT is already there, Access marks it with the _1 suffix to tell the difference. I don't know why you'd want the same table on there twice, but there it is.