IOS – Alert time discrepancies when synchronizing calendars between iOS 6 and OS X 10.8

Mac running OS X 10.8.2 and iPhone running iOS 6. There is an issue synchronizing the alert times of events created on Calendar. Both devices are set so that the default alert times are 30 min before the start of the event.

When creating an event on the iPhone, with alert (say 5 minutes before the start) which is different than the default 30 min, after synchronizing it appears on the computer with alert time equal to the default (30 min before start) and not the 5 min as created on the iPhone.

When creating an event on the computer, with alert (say 15 min before start) which is different than the default 30 min before start; after synchronizing, it appears on the iPhone with two alerts, one equal to the created on the computer (15 min) and another, equal to the default (30 min).

In both cases, after synchronizing, the alert time stays as created on the device on which the event was created. After the second synchronization, nothing changes on either device, although there is discrepancy between the alert times on both devices. In iTunes, the checkbox in front of Calendars in "Replace information on this iPhone" is unchecked.

I am not sure if this started after upgrading to iOS 6 or OS X 10.8, as I upgraded within 1-2 weeks and didn't notice the exact time when the problem started, but the problem didn't exist with iOS 5.1.1 and OS X 10.7.

Any ideas how to fix that?

Edit: iTunes either via USB or Wi-Fi is used for synchronizing. Nothing in the settings has changed since upgrading the computer to 10.8 and the iPhone to iOS 6 (at least not changed by me, the new versions of the operating systems may have changed something).