What does DC voltage reading from a DMM mean?
A typical DMM will only give 1-3 readings per second, whereas a DSO is expected to make potentially billions of readings per second.

The primary consequence of this is that fewer readings per second lets you integrate the reading over time, which is how a DMM can give true RMS readings, whereas a DSO can only give instantaneous readings.

If you slow a DSO down to just a few readings per second and tell it to average the results, you will get something closer to what a DMM gives you. But realize that it is not designed to do this, so a DMM will still do a better job at the tasks it is made for.

Another aspect of the difference is that even high-end DSOs rarely exceed 8 bits per sample, meaning only 256 values per range. Even a cheap DMM will generally give at least 2000 counts per range, and many go more than an order of magnitude higher than that.

These two aspects — speed and precision — are inversely related: it takes time to do a 16-bit or so measurement with high accuracy, so 50000 count DMMs either can only give a few readings per second or they're really expensive. Flipping it around, it's cheap to make a DSO that can do 500 megasamples per second over a 100 MHz measurement bandwidth if you only ask for 8-bit sampling.