High bicarb can be as a result of chronic lung conditions due to the poor exchange of gases but I don't know that that would reflected in someone with well controlled asthma.
It can also be caused by low potassium but I don't imagine that would be the case as the bloods would also show that. The only other things I can think of are prolonged vomiting and/or diarrhoea or just a one off glitch.
Dusty. xxx