There are two main reasons why your registration period may be extended:
First, if you were returned to custody after your release from the sex offense, that custody time does not count towards the registration period. The registration period is tolled or paused if you are returned to custody for a new offense of any kind that results in a conviction. The registration period is tolled or paused if you are returned to custody followed by a violation of probation, post-release community supervision, parole, or other supervision.
Second, if you are convicted of failure to register in violation of PC 290, that also adds time to your registration period. A felony conviction for failing to register adds 3 years to your registration period. A misdemeanor conviction for failing to register adds 1 year to your registration period.
For example, if someone is convicted of a Tier 1 misdemeanor offense, then they would have a 10 year registration period. If they are later convicted of a misdemeanor for failure to register under PC 290 and serve 30 days of custody time, that would add 1 year and 30 days to the length of their registration period. Their new registration period would be 11 years and 30 days, beginning on the day of their release from the original sex offense.