That is not a proper homing to any pet bigger then an insect if you would ask me,
but you don't, so well.......

If you take out the hygrometer,

make a towel damp and wrap it good around the hygrometer,
let it sit for about 10 minutes before reading the humidity.
It should say it is 100% or close to it.

Don't know wich brand you have, but some can be adjusted on the back by a screw.
Otherwise just note what the deviation is and you know what to add to you're readings.

And please, consider a proper viv. for you're frog.