advertising rate

Using beacon_3.40.6 I have an interesting issue with setting adverting rate.
Setting APP_BEACON_INT_MAX to a number less than 0x1A00 results in the correct timing delay (*0.625ms). I also set APP_BEACON_INT_MIN to be the same.
When I set them to be anything higher than this eg 0x1B00 I get the first interval to be correct, but every transmission after this is about double the calculated interval rate.

This does not happen with numbers less than 0x1A00 for the advertising interval. Numbers less than this behave as expected.

Any ideas?