Carb sync question

Greetings from Blighty,
When doing carb sync on my st1100 2000, i use 4 vacuum gauges and i manage to get them all equal. However i have noticed that when i bring revs up slowly to around 3k to 4k the gauges rise for a bit then drop off before rising again. They all stay roughly the same but the fact that the vacuum falls off then rises again is puzzling. Any reason for this?
Thanks in advance .
Ian

Switching from slow speed to main jets...?

normal, as the throttle plates open vacuum is lost as more air enters. The gauges have a slower reaction time than a map sensor on fuel injected engines.

Thanks for speedy replies. I thought it was something along those lines but its nice to have it confirmed by poeple who know. Thanks again
Ian