MLP Season 3.

There's 3 letters in MLP.

There's 2 numbers in "13"

1+3+2=6

6

There's 6 characters in the Mane 6

There's 4 letters in "Mane"

6+6+4=16

1+6=7

There's seven seas.

Sea.

Ocean.

The Pacific Ocean.

The Bikini Bottom is in located in the Pacific Ocean.

Bikini Bottom.

Spongebob Squarepants.

Spongebob makes Krabby Patties

Patties

Burgers.

A Big Mac is a burger.

Big Mac is the name of an MLP Character.

Mac.

Macintosh

Mac OS X

X = 10

The Mane 6, Spike, and the CMC made 10 main characters.

And there were 3 Princesses until S3E13

10+3=13

Could it be that they wanted to make it around their main characters?

Yes.

Twilight becomes an Alicorn in Season 3.

Alicorn is 7 letters

Season is 6 letters

7+6=13

They obviously foreshadowed Twilight Sparkle becoming an alicorn.

Now you know why Season 3 only has 13 episodes.