The reason some car seats are Swedish tested and some are not - it's not a legally required test basically.
Every car seat you buy in EU/UK has to pass one of the legal standards set up in EU legislation - either ECE R44 or ECE R129 - the R44 is an older one. That's law. It's illegal to market a product as a car seat, child safety seat, child restraint etc if it doesn't conform to one of these standards.
When Sweden joined the EU in the 1990s, they had been rear facing their kids for decades already. They already had a test which basically allowed only rear facing seats to be sold, which was their own local standard, the T-Standard. But on joining the EU one of the things they had to agree to was to scrap their own child seat laws and adopt the EU regulation, which at that time and to this day, was less stringent than the T-standard.
So the Swedish road safety authority created a test which would mimic the T-standard, and they decided that although they would accept the ECE R44 standard, they could offer this optional but highly recommended "Plus Test" certificate, that manufacturers could opt to put their seats through, and therefore market them as being just as safe as the old, safer, Swedish standard.
The rest of the world knows nothing of this. Car seat manufacturers throughough the 90s and 2000s continued to make rear-facing only seats for the Nordic market, and forward facing only seats for the rest of the European market (UK, Germany, France, etc). It was not possible at this time to buy an extended rear facing seat in the UK. The only ones were up to 13kg rear facing, and then forward facing to 18kg. Even though there are perfectly legal 18kg or 25kg limit rear facing seats being produced to the R44 standard, some of them in British factories (!) and being shipped to Sweden and Norway and Finland to sell there.
Then came the internet. So parenting forums existed and people started sharing info about car seat safety and about Sweden's propensity to rear face all their kids up until around 3-4 years of age. And some people wanted to import these seats. This was really difficult for a while, then some specialist retailers (like the in car safety centre) started to stock them, though they were very worried that people would install them incorrectly so you had to go and have them fitted in person. It was about 10 years ago that things started to really change and it got much easier to buy ERF car seats in the UK. (I could go on but I've rambled enough!)
So - anyway - back to the point of why - manufacturers do the Plus test on seats that they want to market in the Nordic markets because consumers in general look for that marker there, whereas the majority of UK consumers don't know what it is and wouldn't look for it. And in fact, it's harder to market a purely RF seat in the general European market, because the VAST majority of parents want to forward face at some point, and feel worried about choosing a purely RF seat, mainly because we are unfamiliar with them in general and people like to go with what they know. So they assume that they will FF at some point, even if they have received the message about RF being safer.
As for whether it matters - kind of depends what you mean by matters. It's possible that there are combined RF/FF seats that, when used exclusively RF, will protect just as well as a plus tested one. But having the plus test means that you know that it HAS been tested to that standard and passed.