VARC Sectional scores are too fluctuatimg. Sometimes single digit sometimes around 16 and sometimes around 25. What to do?

I think I am the right guy to answer your question since I have scored 99 percentile as well as 20 percentile in cracku varc sectionals. I would say instead of focusing on these sectionals(let it be any institute) just give VARC sectionals of previous years and then evaluate yourself.
If you want toughness then (VARC 2023=VARC 2022) < VARC 2021.
After this analyse your situation

just give pyq mocks atleast 4 of them take avj marks
diff institution mocks have mocks difficty ranging from too high to too low
pyq will give you a real answer to ur level of preparation

for me too its sometime in 35+ or its 1/3 :)