Power Pivot - Placing measure in a differnet table triggers relationship warning?

Dear all,

Simple question: I have a measure in Power Pivot (Excel 2013) that I initially placed in table Sales. I use it in a pivot and all goes well. If I move the measure to another table, say Customers, Excel generates a warning "Relationships between tables may be needed". Nothing else changes at all, nor the code or the existing relationships. If I place the measure back in the Sales table the warning disappears.

Any idea why this happens? Just placing one measure in one table or another should be completely irrelevant and not change anything at all. Or am I missing something?

Many thanks in advance