You can think of a semi-trailer as a subset of a trailer and as such it can have different regulations applied to it.
It's classified as follows:
- Trailer(Reboque)
- Semi-trailer(Semi-Reboque)
Maybe the Portuguese law on this subject is poorly redacted or poorly structured and that can lead to misinterpretations and/or mistranslations, and maybe that's what happened here, IDK.
And as stated by @mackintosh it's seems odd that no beta tester noticed this error and reported it, as it was one of the first thing I noticed when entering Portugal, with the GPS constantly beeping .
AS for the EC page, as they state that the data shown is provided by the national authorities, it seems to me that someone messed up providing the data by either misinterpreting it and/or mistranslating it.