Mine is a one piece does that make a difference?Well there a few reasons they are better against the wall. One reason is its because they are meant to be against the wall for cosmetics and for function. the pan its self is not designed to support the load of the cistern when bigger people use the toilets. Some toilets even have holes through the rear of the cistern to secure them to blocking or studs.
You also don't need to clean behind them when installed correctly. No dirt can get behind them if they are tight to the wall.
Here's an example of how they are normally secured. If they don't have the holes I use a couple of dabs of silicone.
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