Sequential vs. random values - number of pages in B-tree

Hi!
After doing a quick test:
with sequential values:
create table t01 (id bigint);
create index i01 on t01(id);
insert into t01 SELECT s from generate_series(1,10000000) as s;

and random values:
create table t02 (id bigint);
create index i02 on t02(id);
insert into t02 SELECT random()*100 from generate_series(1,10000000) as s;

The page counts for tables remain the same:
relpages | relname
----------+--------------------------
44248 | t01
44248 | t02

But for indexes are different:
relpages | relname
----------+---------------------------------
27421 | i01
34745 | i02

Plus, postgres does 5 times more writes to disk with random data.
What's the reason that postgres needs more index pages to store random data
than sequential ones?

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