The difference is the change in Mach number over altitude.

And it is more than just the compressibility error in the IAS indication.

The maximum lift coefficient of a wing [goes down with Mach number][1]. While at sea level and 140 KIAS you fly at 21% of the speed of sound (Mach 0.21), at cruise altitude (I guess that means 30,000 ft) the true speed is already 360 KTAS which -- together with the decline of the speed of sound at lower temperature -- translates to Mach 0.63.

In order to estimate the change in maximum lift coefficient, look at the factor maximum lift coefficient times Mach squared: Above maybe Mach 0.4 to 0.5,  **this** is what should stay (roughly) constant. A typical value for a modern wing would be 0.4, so we divide this by 0.63² = 0.397. So your maximum lift coefficient at Mach 0.63 has dropped to about 1.0. At lower altitude the maximum lift coefficient of the clean wing is closer to 1.6.


  [1]: https://aviation.stackexchange.com/questions/64785/aerodynamic-advantage-of-blunt-noses-and-why-subsonic/64811#64811