changing time of event on mobile devices

dear members,

when i add a new event and invite 3 people (for example on 30.10, 10:00h) every Thing works fine.
when i now try to move the beginning from 10:00h to 11:00h (on a mobile device) then the invited People get am message the event ist cancalled and also they get a new event with 11:00h.

doing the same with an Exchange Server everybody get a message event is moved to 11:00h

does somebody have an idea?

beste regards