Fingerings anomaly

Admittedly there may be a better way, but I use fingerings for other uses (row counts in twelve tone music for instance). There do not seem to be any restrictions on the number that is entered except that the digits 8 and 9 do not show. Numbers 8 and 9 are blanks, 18 and 19 show as 1 and 1, 28 and 29 show as 2 and 2, etc. Can anyone explain why this is so?

It’s intended based on the physicalities implied by fingerings and brass instrument slide/valve positions, e.g. as described by Leo in this earlier thread

Perhaps you could use lyrics instead of fingerings to number pitches in your serialist music?

I assumed there was some logic to it. Thank you.