Thank you, Henry, for your helpful explanation. The situation reminds
me of the English simple past "did" and past participle "done". For
95% of verbs these forms are identical and one could assume they came
from the same paradigm. Then one finds those exceptions, mostly
"strong" verbs, in which the forms are different. Or how about the
English subjunctive, which differs from the indicative only in 3rd
person singular forms and for the verb "to be", and is rapidly dying
because people fail to recognise its distinctiveness (so instead of
"if I were... I would" they say "if I was... I would"). Yet there is
no doubt that these were originally separate paradigms and that there
are semantic distinctions which are still made.