Ketones in Urine

There are times when I check my urine for ketones when my blood glucose is particularly high.  For example, this morning before I had eaten a bite, my blood sugar was 406.  I checked my urine for ketones at that time and it showed moderate to high amounts of ketones in my urine.  This afternoon after I ate, my blood sugar was 521 and I checked for ketones again.  On the second check, it was negative to a trace.  Does anyone have any ideas what could cause this to happen?  From going high to moderate amounts to none at all?  Thanks in advance.