Does everyone's app predict a 28day cycle?

Lexy
Only been tracking 2 mths. 
1. Oct-Nov was 31 days
2. Nov-Dec was 33 days 
but next period has put me due as a 28day cycle. Even though I have input my cycles to be usually 30days??!! Worried this is putting me fertile day estimate out as I'm not doing OPKs.
Appreciate any help/advice thanks :)