This behavior still exists in Sketchup 2014 version 14.1.1282 It happens with ARCs and POLYGONs, but not CURVEs or EDGEs.
@tig said:
You don't even need to group the arc, just scale the raw arc in one axis [it becomes a curve] and its displayed length remains as it was originally NOT as it now is.
Only if you explode and weld back into a new curve does the correct length get returned...
This is because an Arc has some special properties - when you scale it it becomes a curve BUT it remembers it used to be an arc - therefore it displays its original arc length [confusingly].
To see this draw an arc, select it and use Entity Info to see its length, scale it in one axis x2, its displayed length doesn't change, it's 'Type' is shown as 'curve'. Now rescale it back on the same one axis x0.5 so it returns to the original scaling - the length doesn't change BUT the type changes back to 'Arc' as it somehow remembers what it used to be before it was scaled.
IF you scale the arc in two axes it remains an 'arc' AND its length is reported correctly.
If you explode the one-axis-scaled arc and weld it into a curve, then gets its length is displayed correctly again, the scale the curve and its displayed length adjusts to suit - so an 'arc' is a special case and if scaled in one axis it will not return its scaled length, unless you remove its 'arc-ness' by exploding it and welding back the bits into a 'true' curve, which will then behave logically...
I think it's a bug that should be addressed in an update...
It's been officially reported to the powers that be.