It could be measurement error if, for example the shim flexed away from the post slightly. Also, since the tube is slotted, it is possible to jam something slightly oversize in a bit, but when you get to the bottom of the slot, the error becomes obvious as it won't go in farther.
It sounds like the 29.8 post is right, and the acid test is that it's a snug, but slip fit in the true size tubing below the end of the slot.
Another solid indicator is the width of the slot when clamped. If the original ID of the tube matches the post, the tube won't close much when clamped tight, so the width of the slot at the top will be very close to that at the bottom.
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