Brettflan/WorldBorder — comment on commit 6b1d7d8cbf0252837eae5074c4a0f50b10a622ca
File:  line 0
By Brettflan:

Using the default padding of 176 blocks on some servers, people were managing to generate an extra chunk or two in places, marring the smooth world edge as seen in rendered maps of the world.
That's the reason for this increase of the default padding. If you don't want or don't need as much padding, you can of course still specify a custom amount of padding in the fill and trim commands.