As another post says, it likely came from http://www.marksteve.com/dtmb (hopefully with permission?)

I am thinking this is probably an artifact of copying the assets from the game, not having access to the originals.

Well,to be honest, flappy birds itself is an awkward copy of a much better game, so I guess its a tradition.

Go to main.js line ~184

In reset() add: SPEED = 300;

and it feels closer to the game.

...copyright?

The creator of Flappy Bird seamed pretty supportive of someone else making an HTML clone of it

https://twitter.com/dongatory/status/431060041009856512