Thank you for the post.
I can confirm this behavior in both FileMaker Pro 11 and FileMaker Pro 12 when running Mac OS X 10.7.4 on my work computer. I forwarded your report to Development and Testing for review. Here are the results of my own quick testing:
FileMaker Pro 11: 1 line = 12 px
FileMaker Pro 12: 1 line = 14 pt
FileMaker Pro 11 and FileMaker Pro 12 both default to 16 when changed.TSFalconFileMaker, Inc.
I would imagine that the precise number of pixels/points per line would depend on the font size specified...