Bottas gained on Lewis 7 thousand on S1, lose 2 at S2 and gained again 14 on S4, coz Lewis braked a little earlier in turn 3 (where he would have hoped to gain the most) after that bad lesson taken on the previous lap. And also lose at turn 10, when he was trying to cut too much the apex unsettled his car after running over that yellow sausage curb, as he was experimenting this a lot since Q1 ...
In contrast, Vettel loses 1,5 tenths in S1, gained 1 tenth in S2 and lose the most in S3 (almost 3 tenths) ...
And last but not least, all 4 top drivers have their top speed between 321 and 323 km/h top speed ...
In conclusion, the gap to Ferrari although at first glance is not too big (+0,334sec) actually it means a lot coz at this type of circuit times are so close to one another ...