I created a Google Calendar and imported events from it. Worked perfectly.
I deleted all of the imported events from my calendar and then emptied the trash.
When I try to import from the iCal URL again, it doesn’t see all of the events. Some, but not all.
I copied all of the events on the original Google Calendar to a new Google Calendar and tried to import it.
Same result — only some of the events on the calendar are offered for import.