Different body shapes behave VERY different in the water. A totally flat bottom gives you very high initial (primary) stability. This feels rather safe. The downside is that if you push the boundaries it will flip instantly, with little warning.

A round bottom has very little initial stability. Every little move makes the canoe tumble sideways. This feels rather unstable - but quite often a round (pear shaped) bottom will give you great secondary stability. I.e. there is little resistance to small tilts, but a very large resistance to bigger tilts.

It takes a while to get used to the more "lively" response of a canoe with poor initial stability - but such a canoe may be just as resistant to tipping as a flat bottomed canoe.

And yes, there are highly unstable round bottom canoes out there... but those are typical a special design for experienced racing paddlers (Those boats are very thin and long, with as little resistance as possible).

Have a look here... http://www.redrockstore.com/canoes/characteristics/stability.htm

and here: wikipedia