Why do masked layers look differently?

[gphpgl]

When it comes to masking the advice is always "press ctrl+R to make sure it look correctly."
Is there a special reason why the canvas and the rendered view doesn't look the same?

[slowtiger]

Processor power and speed.

[Greenlaw]

Variations in graphics card brands/models and drivers can make the 'realtime' preview differ from system to system.

When you press ctrl + r, you're rendering through the CPU which is the only consistent and accurate way to see the final result...but at the expense of speed of course.

[funksmaname]

In my experience, discrepancies in the canvas preview and render only happen when you have nested child layers, who are themselves adding or subtracting from each others masking under a parent group (or similar):

TOP GROUP WITH MASKING
- VECTOR TO BE MASKED
----subgroup vector
----subgroup mask
- GROUP MASK
----group mask vector
----group mask mask

My (probably naive) view is that this is an (undoubtedly extremely complex) bug. It's something we've been mentioning for a long time and I'm sure it's being looked at, but I can't imagine it's a variation in hardware issue...