Err, to be the best allrounder by a country mile, you dont have to be the best batsman, the best bowler, the best fielder, etc. by a country mile either.
Of all the allrounders in question - Sobers, Imran, Hadlee, Kallis, Pollock, Procter, Botham, Miller, Kapil, Mankad, etc., Sobers is easily the best batsman.
Sobers was easily a better batsman than Kallis, who is a bit of a minnow-basher. His batting against genuinely good/great bowling hasnt been consistently as good as Tendulkar/Lara etc.
Kallis is also the best batsman of the lot after Sobers, of that there is no contest.
One reason an allrounder's stats suffer is because they are over-worked. They spend hours batting and then hours bowling. Which is why allrounders tend to have a speciality - most are bowling allrounders- bowlers that are competent bats, Sobers was the other way round- batsman who was a competent bowler.
Just that Sobers' bowling ( 30-ish average) was significantly better than most of the rest's batting and Sobers was easily the best fielder of that lot.
Kallis also averages 34+ if you discount Zimbabwe/BD and against major batting sides of his era ( AUS and IND), he averages 36+
Sobers averages 34-ish against the best two batting sides of his time ( AUS, ENG) and he also played considerably more against them than Kallis has against the best of his times.
Sobers was an excellent pacer but an ordinary spinner and which is why his figures struggle.
Sobers spent far more time in crease than any other allrounder - Kallis being the only exception perhaps( who perhaps spends marginally more time than Sobers in the crease batting but far less time- about half- bowling) and bowled merely 5-6 overs per match less than Warne does !!
If Warne tried to bowl medium pace for most of his time, his stats would suffer too, despite him being an alltime great spinner and Sobers, who was an excellent pacer, suffered considerably while bowling spin, simply to make up the overs.
He used to bowl 30-40 overs after spending 6 or 7 hours batting and that is more effort than anybody has ever put into cricket. If he gets overused and resorts to spin simply to make the overs up, that doesnt reflect on how good a pacer he really was.
Even at the age of 35, he was routinely bowling spin for hours after batting for a whole day or so.
He infact bowled 43 overs after batting for almost an entire day to make his century against India in 1971 at the age of 35.
Sobers was also used as a main bowler, unlike Kallis ( who always has 4 other bowlers to do the bowling and he only bowls as required, as a 5th bowler).