ranked arena match making is not about cp its about your points. you will be against someone near your ranked arena points (e.g. if youre in 3500 you will be against someone in 3400~3600 regardless of their cp).
again its based on your arena points. even wonder why you keep on fighting the same person over and over? reason is #1 as mentioned before. 2# there is no one else queing on your point bracket. until that person gets high enough points and you get low enough then no one else is queing... you will encounter him again and again until youre both out of that point bracket.
i do this as well. i play rank arena on the second or third week of the month coz im trying to avoid the whales who are climbing the ranks during the first two weeks of the season. atm im 13k cp and im just happy to reach master since challenger is whale territory the reward is not worth the effort. yeah i bulldoze my way from platinum/diamond then never bother again. maybe this is the reason on why you encounter high cp players on lower ranks.