The statement is quite self-explanatory
Bottom and top sandstone blocks have different texture.
Notice the cut on top of the sandstone slab.
This makes making aesthetic builds by combining slabs much harder since you can easily distinguish between them
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Could potentially be fixed alongside !MC-137583, if they ever get around to that.
Sandstone slabs just use the same texture on their side as sandstone. It's always been that way ever since sandstone slabs were introduced, hence this is not a bug.