Arthur did not stop after conquering one tribe, but did that, so all the tribes were on the way? He also used the blood mark to make them spread his faith, which added up more devotees to him.
The moment he reached the Sherekan city, he already had over 1 thousand devotees, which were many people. It would surely boost his attributes by a lot the next time the system grants him attributes.
Also, the moment he arrived in the city, the blessing from his system was granted to all of his devotees. As he wasn't near any of them, he did not know what was the blessing, but if it was good for them, then they would continue to worship him, and that would strengthen him.
Now, Sherekan city was incredible. Al though the desert itself and the tribes in it looked poor, and clearly had to struggle to survive, the situation in the city was the opposite.
Tall buildings, sturdy walls, and clean streets. It did not lose to any other big city that Arthur went before.