What am I missing? Explain row estimate wrong

Am I missing something really simple here? If I run my calculations to
determine how postgres determines the number of rows in the explain out
put, my math using the method described in the documentation works just
fine and I get a value which equals waht you see in the explain.

However, I have one table where I don't even come close. No matter what I
set the statistics to and then vacuum analyze, I still get whacky numbers
compared to the explain for the output. Am I just missing something here?
Should I get some sleep and try again?

Here is what I am using for calculations.

set stats back to default of 100

alter table mytable alter id set statistics -1;

ALTER TABLE

vacuum analyze mytable;

explain select * from mytable where id < 12345;

QUERY PLAN

---------------------------------------------------------------------------------------------------

Bitmap Heap Scan on mytable (cost=1444.81..73368.05 rows=76952 width=280)

Recheck Cond: (id < 12345)

-> Bitmap Index Scan on index_mytable_on_id (cost=0.00..1425.57
rows=76952 width=0)

Index Cond: (id < 12345)

(4 rows)

select relname, reltuples::int, relpages from pg_class where relname =
'mytable';

relname | reltuples | relpages

------------+-----------+----------

mytable | 1721143 | 67711

(1 row)

select histogram_bounds from pg_stats where tablename ='mytable' and
attname = 'id';

histogram_bounds

{12,2147,2365,2743,3811,5132,6775,7803,8968,10375,11707,11936,12899,14432,16179,18685,20394,22311,24273,26203,28511,30506,32012,33584,35527,37700,40837,43905,47046,49896,52464,54907,56477,58223,59807,61554,63450,6551

8,66818,68527,69938,71781,73462,74967,76427,78038,80525,82222,83690,85440,86522,88182,89681,90805,93176,95169,97513,99629,101172,103701,105274,107067,108432,109426,111668,114066,116641,118929,122604,125096,127514,1298

12,132095,133867,136484,137017,140121,142244,144270,145203,146996,149353,151230,153037,155255,157480,158277,161314,162013,162054,162298,162928,165418,168405,171089,173359,175749,178139,181864,183148,186855}

(1 row)

My Math .....

( ( 1 + ( 12345 - 11936 ) / ( 12899 - 11936 ) ) / 100 ) * 1721143 = *24521*

*24521* is not even close to *rows=76952*

Thanks for looking!

--
Thanks,

Jorge Torralba
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