Question about Samsung Evo+ microSD UHS-I

Hello digitec-community, Here's what happened. I bought the 128GB version of this microSD. Now I wanted to use it as a memory expansion for my mobile phone (Landrover explore phone, android 8.1.0). Now it shows me the 128Gb card as 256GB with 131GB (systemdata,wtf..) used even though I haven't done anything yet. If I reformat it to portable it shows 128Gb with about 10GB of system data.... I could live with that. But I have now migrated my data from the internal storage to the SD, (I only had 10GB free) but the data has somehow only been copied and not moved. So I still only have 10GB free but an additional 40GB on the SD card. This now says 171GB of 256GB used. (it's still a 128GB, btw). Can someone explain to me what the hell is going on? Have I fundamentally misunderstood/wrongly done something? Thanks in advance