Hot-forged or cast cranks are much more amenable to being straightened, because their grain structure allows planar sliding within the grain (as opposed to a forging's disrupted and thus more interlocked grain structure).
A cast crankarm will generally have larger cross sections so that bending or fatigue don't occur at too low of a force level, making it feel stiffer at stress levels below the yield point.
Note that some aluminum alloys and casting processes produce parts that are less tolerant of yielding (and of corrective yielding) than others!