Inconsistent Head Sizing on bolts?

[Arnold]

I have run into this a few times recently. Metric bolts around 15-20mm bolt head size(socket size). No..the heads were not rusted/rotted(been there done that) and the fit was just too sloppy to use metric sockets..had to use sae sockets.
 Years ago I laid out my sockets,wrenches so that they are in order of actual size..I just used whatever fits best. But a few times lately something has caught my eye and I said..hey wait a minute..metric bolts and the head size is waay off.

[Harry]

I ran into the same thing. SAE heads on Metric bolts.
It was way back when GM was changing from SAE to Metric.
Maybe around 1980?

[enjenjo]

I still run into it. I had a tool recently that had imperial bolts with metric heads.

[Charlie Chops 1940]

I've run into some real buttery small metric bolts and machine screws lately...box store stuff mostly. Don't use much metric fasteners unless they are for an already threaded hole. Haven't noted the head discrepancy yet.