GT4:p had bundles here in europe, which sold like crap due to them being bundled for 300 euros at a time that the PS2 was about 250 euros. Even then, GT5:p sold amazingly without the bundles, I don't think that you can't account 2M difference on bundles only, plus we're not even taking into account DD from the PSN which can easily be 1M more.
You're right about the competition not matching GT sales, yet it was one of the main reasons why GT4 lost so many sales, especially in NA territory, because Forza had online, and it was a huge deal for NA even at that time. Hopefully GT5 will correct this.
NFS has always been catered to a different racing crowd, most of the people that I know, love the GT series and hate the NFS since NFS:Hot Pursuit. That's because they went towards a totally different direction since that game, alienating almost the entire fan base that they had been constructing since the PS/Saturn days.
Current PC Build
CPU - i7 8700K 3.7 GHz (4.7 GHz turbo) 6 cores OC'd to 5.2 GHz with Watercooling (Hydro Series H110i) | MB - Gigabyte Z370 HD3P ATX | Gigabyte GTX 1080ti Gaming OC BLACK 11G (1657 MHz Boost Core / 11010 MHz Memory) | RAM - Corsair DIMM 32GB DDR4, 2400 MHz | PSU - Corsair CX650M (80+ Bronze) 650W | Audio - Asus Essence STX II 7.1 | Monitor - Samsung U28E590D 4K UHD, Freesync, 1 ms, 60 Hz, 28"
