Another possible factor could be the fact that doctors tend to make more mistakes during operations on Fridays compared to start of week.
STEMI centers (this is picked up by EMS and these ambulances are redirected to appropriate centers) have 24/7 cath lab coverage and any major one will have an ER bypass even during afterhours to expedite care.
In fact more and more hospitals (and all the major ones) announce a “Code STEMI” overhead either when the ambulance is dispatched or as soon as the ECG showing ST elevations is discovered in triage/ER to activate the team and reduce door-to-balloon time.
Edit: aha--https://www.nature.com/articles/s41467-020-15432-4
When I was an intern I was 100x better than I am at this point in my career.
The basic atrial fibrillation and STEMI is something any doctor can interpret with confidence (I used myself as an example as I'm probably the least competent because I haven't looked at an ECG in 10 years). I doubt non-cardiac surgeons are much better on average as they don't really look at these themselves that often other than for basic things.
An ER or general internal medicine physician is expected to be competent in more advanced but common stuff like bundle branch blocks, left ventricular hypertrophy, non ST elevated MI.
Weird arrhythmias or conduction abnormalities is really only for cardiologists, and even then typically a subspecialist electrophysiologist.
US/Can healthcare systems still pay a cardiologist a couple of bucks to "finalize" the interpretation whenever they get around to it. It's a bit ironic, someone could have an MI on Friday, get treated and discharged and we're still paying someone on the Monday to read 40 ECGs (note these would have been acutely interpreted by the cardiologist treating the patient, most places have rules against self-referrals so you can't formally interpret anything you order yourself).