Samsung Canada, has definitely been getting a lot better at providing the Monthly Security Patch Updates.
It's not perfect, but it isn't unusual to see a delay in the Security Patch OTAs.
You should see the April Security Patch OTA for your device around April 12th.
Is the security update schedule for Galaxy S22 Ultra and S23 Ultra same for all Canadian carriers (Fido is yet to update their schedule, hence it's quite mystifying) or it differs from carriers to carriers?
It doesn't matter which Carrier you are with. All the Software / OS / Security Patch OTAs are coming directly from Samsung Canada's Servers.
The OS Update Schedules released by the Canadian Carriers, are based on the information they receive from Samsung Canada. The Canadian Carriers have no control over what & when updates are released.
The Carrier's OS Update Schedules are really just a courtesy more than anything else.
We don't know that for sure. Rogers OS Update Schedule may, or may not be accurate.
If in fact it's the April Security Patch OTA, whatever was in March's Security Patch will be included in the April Security Patch. You aren't going to miss anything.
As for any explanation? Who knows. There's likely a good reason for it, but we may likely never know exactly why.