I don't think you understand how the Patriots grade NEED. They very likely graded OG, DT, LB, and CB as having the same NEED. So then it came down to everything else about the grades. How they grade the prospects potential as well as how the prospects total grade is against the players of the same position on the team.
If they had Strange as a higher grade than the other players remaining on their board at those positions, they aren't going to ignore that.
Your statement is wildly inaccurate. Karras got a 3yr/$18M contract. The 29th pick last year got 4yr/$11.9M contract. Let's say the pool went up 20%. That's still a 4yr/14.28M Contract. That's significantly LESS than what Karras got. the 29th pick last year had a 2.17M cap hit. So, Strange will have around a 2.6M cap hit. Karras' cap hit is $5M this year.