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Hey Grace,
First things first, your A7R5 has a max sync speed of 1/250th, so with your previous strobes (which ones?) you were getting an unexpected bonus.
I wouldn’t worry about your fibre optics: if they’re good enough to let sufficient light pass through and fire the strobes, that light will still travel at 300,000km/s.
Variations on actual max sync speed do occur (up or down), and they’re mostly due to the flash trigger.
Down example: when I reviewed the Nikon Z8 with Nauticam’s standard flash trigger, I found it wouldn’t let me achieve the Z8’s 1/200th sync speed (black band on the top), but the Nauticam TTL trigger didn’t have that problem.
Up example: some people have noticed achieving slightly more than the camera’s max sync speed, with specific flash triggers/strobes, you’re one of them 🙂
However, it’s possible that you actually didn’t notice the black band when shooting at 1/320th. If it was wide-angle, you likely didn’t have anything to light in that top area of your frame. Or maybe you were in APS-C mode, then the A7R5 sync speed can go to 1/320th.
If not, the difference you’re observing might come down to the Retra being more powerful, and/or shining for a different duration when delivering their flash (strobes vary a LOT in this regards). Everything else being equal, my Z8 sync speed issues where showing up more with the Retra Pros and Ikelite DS230 (both powerful strobes) than with the Inon z240s.
Hope this helps, but it looks like your camera performs normally, and you’ll need HSS if you want those higher sync speeds (assuming your flash trigger has HSS support).