Hi Hawk37,
Note that I did say, "at a constant level". A series of constant levels is typical of an aircraft conducting periodic step climbs to be at the level closest to optimum, thus, initially the weight is above optimum, decreasing to optimum weight for the level, and then further decreasing to a weight less than optimum for the level until burned down to the weight for the next step climb.
At the initial high weights (when the weight is above optimum), Angle of Attack is high (due to the high weight) resulting in greater than desired acceleration of air over the upper wing, resulting in slightly slower Mcrit. As a result, because MRC is closely related to Mcrit (always being ABOVE it!), flight is at a slower 'than might be expected' Mach Number. As weight decreases, required AoA decreases, acceleration of air over the wing is less, thus the aircraft may be flown faster (increasing Mach Number) for the same wave drag, and MRC increases slightly as the weight decreases. After this very small increase with reducing weight, MRC Mach Number then steadily decreases with reducing weight, as might be expected.
As LRC and all Cost Index speeds between MRC and LRC are inexoribly linked to MRC, the same trend is noticeable at these speeds also.
Actually, it's quite noticeable in the cruise control tables from many AFMs, the initial small increase is usually only of the order of M0.02 or so.
Regards,
Old Smokey