Correct (almost!) that there are only two major mapping companies and the maps are indeed similar, however, the difference is the ability to process via a preferred road type for the calculated route, rather than simply a track that does go from a to b. If you have ever seen a HGV Haulage Road atlas, then you will understand better, as although the maps are similar, only certain types of road are deemed navigable.
If you have never been stuck with your sat nav - excellent! However, I could give you details ona daily basis of the lorry drivers down here who are guided down a two way road for a few miles to discover that there are no passing places if there is any traffic coming the other way. Sure you could get an Artic down the road with care, but not if a cyclist was coming the other way and you did`t want to run them down. A portable sat nav would see this as a viable route, wheras the In Car one would see it as an optional route if the larger road and preferred route could not be found. Thats the difference.
And Yes Jules, you are correct about the "Learning" most In Car systems will build up a certain amount of Local Knowledge of the roads you travel down most and the types of road you use most frequently and use this for future route guidance. My Personal favourite is the Blaupunkt DX-V - and for the first three months of use was one of the worst user interfaces around, but now it has built up knowledge of my style of driving, its now coming into its own. The Panasonic is also very similar, after a few months use, it suddenly becomes exceptionally good at guidance. I ran a Tom Tom, a Garmin and a Blaupunkt DX-V simultaneously on a 3200 mile route I drove between the UK and Poland and gave some stunning results - but that is another story completely.
As I previously said, I`m not knocking the Tom Toms, as they are very good at what they do, but simply do not compare to a good In Car system.