Thanks for the info, yes I did forget the difference in expressing millibars and inches of Mercury. I had never heard about the "1000" ft to 1inch ratio.

In any case, (aneroid barometers) or altimeters do suffers from artificial as well as natural variations in air pressure. I think the cateye may suffer from "artificial pressure "fronts"" that develop when cycling on "unsettled" gusty-windy days. The same device may work much better when strapped to a wrist of a person at walkspeed.......