The short answer is that Tcl (like most comuter languages) doesn't do fractions; it only knows about integers and floating-point numbers. Hence expr doesn't "evaluate" fractions, but the way you would ask it to compute a quotient may look like as it evaluates a fraction.
[Explain idioms of "computer arithmetic".]
Sometimes, Tcl's expression handling can be a little bit unexpected to newcomers:
% expr 1/2 0
This is because the above code asks for integer division (this is true in many other languages too) and that (conceptually) contains a round-down-to-int of the result. (The remainder operation, %, is often useful when you're working with integer division).
% puts "1/2 is [expr 1/2] remainder [expr 1%2]" 1/2 is 0 remainder 1
If you really want to end up with a half, force one of the arguments to the division operation to be a floating-point value:
% expr double(1)/2 0.5
See expr problems with int for more on integer division and how to avoid it.
rdt : 2005-01-27 : This is a property of integers that can take on values of 0,1,2,... and therefore have no values between 0 and 1. However:
% expr 1.0 / 2 0.5
show the expected value. Whereas in a calculator, most math is done in floating point, in Tcl, floating point math is only performed when you explicitly say to do so.