A CurvedAnimation can also exceed the 0.0 to 1.0 range,
  even if the AnimationController doesn’t.
  Depending on the curve selected, the output of
  the CurvedAnimation can have a wider range than the input.
  For example, elastic curves such as Curves.elasticIn
  significantly overshoots or undershoots the default range.

Tween

By default, the AnimationController object ranges from 0.0 to 1.0.
If you need a different range or a different data type, you can use a
Tween to configure an animation to interpolate to a
different range or data type. For example, the
following Tween goes from -200.0 to 0.0:

A Tween is a stateless object that takes only begin and end.
The sole job of a Tween is to define a mapping from an
input range to an output range. The input range is commonly
0.0 to 1.0, but that’s not a requirement.