According to Wikipedia for the 3 Spurs managers (for ALL games not just league games)
Tim Sherwood - 28 games W14 -D4 - L10
So a "win" ratio of 50%
Villas Boas - 80 games - W44 - D20 - L16
So a "win" ratio of 55%
Pochettino - 90 games - W45 - D23 - L21
So a "win" ration of 50.56%
So in fact his win ratio is the same a Sherwoods, but Villas Boas has a better "win" ratio.
However the win ratio is not the only measure.
So for example if a manager gets knocked out the 3rd round of the FA cup that is considered a failure, but if you lose in the final itself that is a success.
Both are a single defeat, but one can get you the sack (3rd round), the other a partial success.
And with Potechinno they are now 4th in the league, and still in the FA Cup and Europa League, and so far it can be considered a successful season.
p.s. I felt Sherwood was unlucky to lose the job at Spurs (maybe he took over a "good" team), but he was somewhat of a failure at Villa (where of course he had a rather poor team).