If you Select a single Arc then the 'Entity Info' report is its 'true length'.
If you Explode an Arc into separate Edges, or if you have a welded 'Curve', then 'Entity Info' reports the total length of all of the individual 'Segments'.
So an Arc and an Exploded Arc will report slightly different lengths - the exploded version is always going to be somewhat shorter because each segment will 'cut a corner', whereas a true-arc has a summation of a 'infinity' of 'tiny-tiny' edges [2Pir*included_angle/360] - the effect of Pi is to give a result that can be refined more and more, but never quite gets to an unchanging value, as it can always be improved upon - of course in practice these tiny fractions of an inch are unimportant and so we 'round-up'... Depending on the accuracy of what you want to measure/buy/cut etc the 'segment summation' will probably do, because only a few mm are involved in quite large objects between true arc-lengths and summed segment-lengths...