The meta develops over time as the event goes on. Heroes like techies got a pretty significant buff with their E now providing an aoe stun. Heroes that didn't get any significant buffs but are showing up in the meta during the event are probably because they fill a role to counter something in the enemies draft. For example, even though storm has not been given any major buffs in 7.33, midlanders that dive the backline have been very popular at the moment and storm fills that role very well.
As the event goes on, the pool of heroes will get larger. Usually groups have less heroes picked than main stage historically I believe.