Tuning 25.5" scale Open Back to Minstrel Tunings

I tried to tune my 11" open back banjo down to a 'minstrel' eAEG#B and then lower still to (Briggs) dGDF#A.

I loved the sound but...

I found it harder to get the right note on each string as it got lower. The tuner would sometimes show a slightly different note on subsequent plucks, F then F# then F again.

Is this because the scale length?

Is it because of the wire guage? It's normally tuned double C or Open G.

Is it because it's a cheap asian banjo?

Any ideas would be appreciated...