When you pop above 45K here tax starts to get excruciating (40%) so that is probably why the upper limit is sat there (I get 8% year on year increase in my current job and it's barely noticeable due to tax...)
E.g http://www.thesalarycalculator.co.uk/salary.php
(figures in GBP)
30k -> 22.6k (25% total employee tax burden)
45k -> 32.8K (27% total employee tax burden)
90k -> 59.4K (34% total employee tax burden)
135k -> 86.0K (36% total employee tax burden)60K -> 41.7K (30% total employee tax burden)
There is a sudden big leap (of an extra 1%) in that particular range before it settles back down again.