I get this too. As a new MI user, the scale bar options seem extremely limited and, to be honest, crippled and ridiculous!
We do have a tab file that contains a scale bar with a pre-set width and if you set your map frame and scale bar frame manually to the same scale, then it is accurate. This is a funny little workaround though, since surely PBS could have coded up something similar years ago!
How do others do scale bars?