It's not a bug.

If your flag weight reaches 500, the calculation turns from linear to asymptotic, eventually reaching 749.99999 (or maybe 750, if you're lucky).

See also: [What is flag weight?](http://meta.stackexchange.com/questions/80170/what-is-flag-weight/80178#80178)

Maybe it should be rounded, but this display gives you an idea of what you're heading for, if you keep raising great flags :)