It seems that MS Outlook Mobile doesn't handle calendars correctly...
I suggest you to try to use GMAIL mobile app and see if it works better
Gabriele Maoret - Head of SysAdmins at SERSIS
Currently manages 6 SmarterMail installations (1 in the cloud for SERSIS which provides services to a few hundred third-party email domains + 5 on-premise for customers who prefer to have their mail server in-house)