The ICC average rankings are just one way to gauge players.
Tendulkar, in tests, pretty much always averaged between 50 and 57, so it's surprising he isn't ranked higher. It's probably because he rarely dominated a full series, never got 500 runs in a series, and never achieved 900 or more ICC points. Lara in contrast oscillated between huge highs and very low lows. Meanwhile, someone like Viv Richards spent most of his career ranked 1 in the ICC rankings. And Sobers spent a decade averaging in the 60s or 70s.
Tendulkar's test runs in the 1990s came against decent bowling attacks. He often dodged Warne and Mcgrath playing together, but had to face Walsh, Ambrose and Donald and wasn't bothered by any of them, so I doubt the ICC rankings are punishing him based on the quality of opposition faced.
I reckon Tendulkar was a better Test batsman from 1990 to 1999 than Lara. Tendulkar scored centuries on his first tours to every country, and was doing this all as a skinny, fresh faced kid. Tendulkar had his s**t together, mentally and technique-wise. Lara was more of a show-off, liked to do impossible stuff, and eventually had a big meltdown in the mid 90s. His test career nosedised and only started reassembling itself in 99.
Lara probably gets points from having to play the Aussies alot. And unlike Tendulkar, he always faced them with their full strength bowling attacks. What's remarkable is how good his record is against the Aussies, despite being repeatedly cheated out by them. Ian Healy cheated him out in the 92 series (stumped with no ball in his gloves). Then the Aussie tour to West Indies in 95 saw Steve Waugh cheating Lara with a bumped catch. During the 2005 WI vs Australia series, Lara was also wrongfully given out 3 times in 4 innings, prompting the ICC to formally appologise about the quality of umpiring. The Aussies always pulled this crap on him; I reckon he missed out on 3 more easy centuries vs the Aussies if not for cheating.
Also, these ratings dont take into account ODI games, where the "complete package" qualities of Tendulkar become even more evident.
Lara had the better ODI career from 1990 to 1999 - the best ODI bat in the world during this period - but then sucked in ODIs from 2000 onwards due to various reasons (he moved around the order too much, seemed to lose interest, and his top order batted slow as hell).
Tendulkar, in contrast, was okay in ODI's from 1990 to about 1996, but then blossomed and became a consistently excellent ODI batsman from then on to his retirement. Opening also allowed Tendulkar to pile on huge amounts of ODI runs. Statistically he's one of the greatest ODI batsmen. On the flip-side, only 6 of his ODI centuries came outside the UAE/Subcontinent. He averages only 37 playing non-minnows outside those conditions.