How do the boost points work ?

Please, remind me how those boost points are supposed to work, because I thought that each time they should go higher but I have quite a few horses that seem to be stuck. Their boost points are for example: 11, 11 or 13, 13 and one horse just received 19 boost points for the third time in a row.
Shouldn't it be: 11, 13, 15, 17, 19, 21 ????

^{According to the original news post, they only increase their stat boost totals if they go up a grade in that time. So Regional earns more stats in a boost than a local grade horse does. (News Post)}

Thank you for the link

Boost points are linked to grades, so the horse gets that many stats each boost period depending on what grade they are.

Novice: 11 stats
Local: 13 stats
Regional: 15 stats
National: 19 stats
International: 23 stats

Thank you, I'm going to put it in my notes ! I was monitoring each horse recently because I was trying to get to those numbers but got confused by duplicating numbers - now I understand :)