I have 3 level 10 speed ruins on Kihaku, but I have challenged this monster as an enemy with speed ruins of 3 9's or recently a level 8, and 2 two level 6's and he went first. Plus he was a level 105 and my monster is level 110. Why do they go first with lower levels?

It doesn't matter the level. You should have checked their speeds to check who will go first. Maybe some monsters were using team speed runes to boost main denier's speed.

Rune Guardian? More team speed runes on the opponent?

@keki14

Thank you Keki. You always answer/resolve my question.

I think also checking to see weekly 'buffed monsters' where a monster is getting a free temp stat buff 10%-20% can mess up your math and presumption of being faster