Ok, my guess was right. I ran a program three times using a 2hz flow signal with no wifi UI connection. The total water consumed reported in the log was 1802, 1802 and 1801. So the wifi ui is definitely hosing up the flow time measurement capability of the opensprinkler program.
BTW I’m running buster Lite.
The wifi connected UI offered a hint as to the problem by closely watching the flow count updates being reported. Every so often the number would jump through two updates in a row in less than a second.