Well, I thought by now someone else would have answered it. But, I'll try. I'm not familiar with this chip and board, so what I can tell you is pretty limited.
In short, you have a 3.3V micro-controller and generally speaking you don't hook 5V signals to them. There are some 3.3V parts that have "5V tolerant" inputs, but that only makes sense for digital inputs so far as I know.
Barring anything unpleasant happening, the behaviour I would expect to see with hooking 5V to the input of a 3.3V ADC is that it would clip at the high end value. So yeah, for 10-bit resolution, which appears to be the default for the Nano 33 BLE and with VDD as the reference, which also appears to be the default, I would expect to see a value at or near 1023. Up until the point that you fry something that is.
I don't know why you're seeing lower values, but since you seem to be operating it out of specifications, I'm not sure that really matters why. If you want to measure 5V with your 3.3V max ADC, you should involve a voltage divider.