BBT SLOW RISE? Chart included.

I hope this isn't to confusing.  It seems for the last 3 months I will have a small rise (.44-.55) rise in bbt after ovulation around CD14-15. Then almost like clockwork my bbt rises VERY slowly from there with a small dip CD18-19 and then a very sharp rise again at CD21 (DPO 7).  My cycles are 26-27 days. Anyone have a similar bbt situation?  I've read something about some women being "slow risers" but am confused as why?  ​