Actually, if you look at the second photo, it looks like they are stitched in spirals; no joining of rows.
The look is achieved by single crocheting into both loops. Very standard for amigurumi. If you crochet in back-loops-only, you'll find your amigurumi getting a sort of spiraled ridging all along the outside.
If you find your project still looking different than those in the photos, perhaps your amigurumi is inside out. PlanetJune illustrates right/wrong side of amigurumi very well here- http://www.planetjune.com/blog/which-is-the-right-side/
Working in spirals doesn't create a very noticeable curve in stitches at all. Keep in mind that joining rows would give a very straight appearance to the stitches, but will also create a noticeable seam.