Too much humidity?

Hi everyone,

I finally brought my male common boa baby home today.
He is settling well into his set-up, but I am a bit concerned about the humidity. It is saying 89%, and I can't work out why.

He is currently in a 9ltr RUB with airholes. This is only a temporary home until he gets bigger, as he is a small baby.
I've put a little snake aspen bedding in there for him, a hot hide and a cold hide, a waterbowl.
The hot temp reads 30 degrees celsius, the cool side 23 degrees.

I don't understand how the humidity could be as much as that, the bedding is nice and dry. Is this going to pose a problem for him? Is it too much? The room itself is not humid, and is nice and warm as I use a small oil radiator to help the ambient temp.
I just really want to make sure everything is safe. Is this just the nature of a RUB tub? It is the first time I have used one. There are plenty of ventilation holes along the side and a few on the lid.

Thanks for any help!

Well, that does seem odd to me, especially if the aspen is dry. Technically that humidity is ok as long as there is air flow and nothing is molding, but it makes me wonder if it is reading correctly.

Maybe because it's a new bag of substrate? When I open a bag of Forest Floor or new prepared Repti Chips they are so wet. After a couple of days the humidity will lower~

Hmm it is the same bag of substrate that I've been using for my ball python.
Perhaps I will switch the thermometer/hygrometer for a different one to see what difference it makes.
I will probably do it tomorrow because I don't want to bother the baby.

The two thermometer/hygrometers in the tub are brand new, and they are the battery type, not analogue.

The water bowl is inbetween where the hot and cool hides are, so it might be due to that. I will move it right into the cold corner to see if that helps.
I just want to know he is safe. Thanks guys :)