There are more episodes in some seasons, because of the difference in when they premiered:

Most of the seasons (1-9 and 14) premiered in the fall (September/October) and ended in the second half of the next year.
These were weekly episodes.

Season 10-13 started in the summer (July) and ended in the second half of the next year.
For the summer there were extra episodes ordered.
With the exception of season 13, the episodes were then aired daily (Mon-Thu) in July and August.

Season 10: 44 episodes
Season 11: 45 episodes
Season 12: 40 episodes
Season 13: 40 episodes

Note:

Also take into account that sometimes episodes were not aired regularly: a week or a month might be skipped.

Although season 13 only had weekly episodes, including the summer, it still had 40 episodes because it aired from July 2013 - July 2014.

@wonder2wonder said:

There are more episodes in some seasons, because of the difference in when they premiered:

Most of the seasons (1-9 and 14) premiered in the fall (September/October) and ended in the second half of the next year.
These were weekly episodes.

Season 10-13 started in the summer (July) and ended in the second half of the next year.
For the summer there were extra episodes ordered.
With the exception of season 13, the episodes were then aired daily (Mon-Thu) in July and August.

Season 10: 44 episodes
Season 11: 45 episodes
Season 12: 40 episodes
Season 13: 40 episodes

Note:

Also take into account that sometimes episodes were not aired regularly: a week or a month might be skipped.

Although season 13 only had weekly episodes, including the summer, it still had 40 episodes because it aired from July 2013 - July 2014.

damn! that's crazy so all their later episodes have over 40 episodes? Im binging like crazy and still on season 10 lol.