Others may disagree, but I think Engel's rarity ratings are best thought of as falling along a spectrum from "these things are all over the place" to "these don't come up much", rather than as a strict numerical population report. There's just no way to tell how many of any given card are out there.
One other thing to remember is that there are a zillion different sets, many of which were produced in small quantities.
Consider this: imagine that there were ten million sets made, each of which is R5. In that case, finding some R5 card or other would be no big deal. (Although finding some particular one would.)
The real case isn't quite like that, but since there are a large number of sets (far more than American cards during the same years), getting some rare card or other isn't very surprising. (Although if you know of several copies of an R5 card, that is some indication that Engel's numbers for that card are off, which, as I said at the beginning, probably shouldn't be a surprise.)
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