[Tilesathome] Strange rendering of straight lines

[Maarten Deen]

I'm a bit puzzled for the reason why this gets rendered as it does:
<http://informationfreeway.org/?lat=-84.951&lon=-144.66&zoom=12&layers=B0000F000F>

Tiles are
<http://server.tah.openstreetmap.org/Browse/details/tile/12/402/4083/>
<http://server.tah.openstreetmap.org/Browse/details/tile/12/402/4082/>
<http://server.tah.openstreetmap.org/Browse/details/tile/12/401/4082/>

In JOSM, this is a straight line. Why does the render mess this up so much?
Two tiles have a turn in the coastline, and the middle tile is not in line with 
the outer two.

If you look in JOSM with the slippymap plugin, the rendering is way off of where 
it should be.

And the coast is riddled with these tiles.
Is this some side-effect from being close to the pole?

Maarten