Why is this Marsh puzzle solution wrong?

Ok, I'm new to the game (under an hour) so bare minimum of spoilers please.

I'm trying to solve the marsh puzzles and from the "tutorial" set, I picked up the following rules...

• The outline must include all the symbols
• The shapes defined in symbols must fit in the enclosed area with no overlap and nothing spare
• The symbols don't have to be on top of respective shapes

This seemed to work for all the tutorial puzzles, but then I tried these rules out on the first puzzle, to extend a bridge.

This didn't work

But this did

Why? What additional rule am I missing?

I found the working solution by trial and error and want to understand before progressing