I don’t know the exact mechanics of how layout happens here, but the two things are not equivalent: if you put rotate() in a cell, then the cell’s content is rotated. in the show rule you rotate it, which is the whole cell. I would say that in general, the cell should not be rotated. I could e.g. imagine that this leads to the top border actually being the left one; you can test whether that prediction is correct.
You can however (unless there are specific limitations with table cells, which may be the case) write a show rule that replaces the cell’s body. The technique is basically the same as the one used in How to avoid nested enum items breaking at the end of the page? - #2 by SillyFreak or How can I manipulate content values? - #2 by SillyFreak (bottom code sample).