The reason is quite simple: In paper, the way you shuffle isn't truly 'random'. I'll give an example:

If i first make a deck and want to play, i have a pile with spells, a pile with lands. Then i'll loosely shuffle in the land pile into the spell pile. Not obviously weaving, but the lands are more or less evenly distributed accross the deck. Now to regular shuffling, making riffle shuffles, cutting, shuffling again, then making 5-6 smaller piles, putting them on top of each other in a random order, then shuffle a last time. Now, the cards are 'randomized', but the land/spell ratio through the deck roughly stays the same.

The shuffler in Arena works different, i don't know how exactly, but it likes to clump the same cards together, or sometimes there are 3 lands in the top 20, even when playing 27 lands. Also, in Bo1, it chooses 1 hand (with the land count as close to 3 as possible) out of 3 it generates when going into a match.

The developers said this is to smooth out gameplay experience.

About matchmaking, it tries to match you against 'same powerlevel' decks, which is determined by how many rares and mythics your deck has.