Cycle Length Start Day

I had always you calculate your first day of your period as the start of a new cycle but I just realized on here when I put in my cycle is 33 days, it counts 33 days from the end of my last period. So, it says last month my period was from July 21-24 so 33 days should be Aug. 22 but it says I shouldn't start until the 26. This is really confusing me about whether I'm havin implantation bleeding or starting. Can anyone explain this for me? Thanks!