Tytus Howard has improved his Pass Blocking Grade each game this season, per PFF Week 2: 47.1  
Week 3: 57.9  
Week 4: 75.4  
Week 5: 76.7  
Week 6: 77.5  
Week 9: 80.2  
  
His pass blocking grade of 81.3 since week 4 is the 11th highest among tackles in the NFL over that span.  
  
He is the highest ranking rookie tackle over that span (Jawaan Taylor, JAX, is 29th among tackles with a 70.5 grade), and the highest ranking rookie offensive lineman over that span (Tytus Howard is 19th overall, second is G Dalton Risner, DEN, at 29th with a 79.7 grade.  
  
As far as why his overall average is higher than his performances, I believe the overall average is normalized to the league, whereas the game performances are not.