Synced Simplex 2904s?

My high school’s strobes are 2904-9101s with a couple 90s 4904 strobes. I’ve recently noticed that the 2904s are in perfect sync while still running off an original 4002 and I know they don’t sync under normal circumstances. It’s the same way woth the 2904s on a 4001 at the elementary. How is this even possible?

Given the 2904-series was made years before synchronization started to become a thing I have no idea how they could possibly all sync together.

probably one of these:
A) The strobes have potentiometers in them to control the flash rate, and they are all tuned to the same rate
B) the strobes are designed to flash at the same rate, but something wears down in each of them that changes the rate.
C) the strobe discharge circuit was modified to accept sync.

That’s exactly what I said. I hope to try and get a recording from the middle school wing (one hallway) which has four (one in-between each set of rooms) 2904-9101s paired with 2901-9840s of the sync.

I originally thought they were on the horn circuit because they flash with each pulse of the horn but they flash during the long pause during 4002 code three, which in normal cases would cut power to the strobes. In what you listed it’s probably number one. They haven’t been removed since they were put up, in I believe '82 when the school was redone. It was built in '75 and didn’t have a fire alarm system until then.