Could it be that the OP earned Gold, then during the Gold membership year requalified for Gold?

For example, if you are Blue, and earn 1200 points within a two month period, you will retain Silver for pretty much two whole years. When you reach the 600 level, the membership year resets and zeroes out your TP balance, then you quickly achieve another 600 points requalifying you for a new Silver card once your first one runs out.

In the OP's case, if he had earned 600+1500+1500 within a few months, he could have expected to receive a renewed card once the first Gold card ran out.

Maybe this doesn't quite work with the 18month time period the OP mentions, but add on those extra months it takes BA to send this stuff out, and you never know!