sync alert: will change more than 25% of your calendars

I am trying to sync my iphone to my mac via itunes, and I keep getting a sync alert: "syncing with [name] iPhone will change more than 25% of your calendars...." Specifically, it is showing that it will add 13 calendar items, modify 2, and delete 1273. I checked the details, and the 1273 is essentially saying that it intends to wipe out my entire calendar history, which I do not want to do. I do want to add 13 items and modify 2 ... that's the point of syncing. But how do I keep it from wiping out what's already on my calendar?

Through other searches, I found and tried the following tips (to no avail):

I checked the settings on my iPhone and under "Calendars" changed the sync setting to "All Events" (it had been set at 1 month).

I tried unchecking "sync calendars" in itunes, syncing, and then re-checking and re-syncing.

I have two work computers, one at the office and one at home; the older hard drive was copied onto the newer one when I got it, so the calendar items started off the same. I sync the iPhone regularly to both computers, to keep the calendars all updated. I want to keep all of my calendar items in all three places.

(BTW: the iCloud is not an option for me. Some of my calendar items contain student names and the information is considered private, protected by federal law. I cannot upload this information to the cloud.)