Some mixers have bizzare features and design flaws. I should imagine this is one of them. Maybe to some people this is helpful?
You should have a rough idea of how loud your records are but to help you out when you cue you could siwtch the cue monitor between the channel that is playing and the channel you are bringing in. That will give you help in matching the levels before you mix.
I have an Ecler Nuo5 but for a budget mixer you can’t go far wrong with this:
Used these quite a bit and they are a top little mixer. Nice build quality and very simplified. Only £189 now. They used to be about £500 4 or 5 years ago.