I don't get this but I had my period of June 21-27 and mines a 28 day cycle and but on do it says my…

I don't get this but I had my period of June 21-27 and mines a 28 day cycle and but on do it says my next period is July 19th but that's over 28 days so my period should be comming on tomorrow right bc that's the 28th day of my cycle