[Genome] Difference in nucleotide counts

[Li, Jianying (NIH/NIEHS) [C]]

Dear admin,

I am currently working on a project and need to get the total count of
nucleotides of human hg-17. After I finished the counting, I compared
with the statistics listed on the genome browser's page:
http://genome.ucsc.edu/goldenPath/stats.html#hg17 

Chr	Assembled Size	Sequenced		I counted
1	247249719		224999719		245522847
2	242951149		237712649		243018229
3	199501827		194704827		199505740
4	191273063		187297063		191411218
5	180857866		177702766		180857866
6	170899992		167273992		170975699
7	158821424		154952424		158628139
8	146274826		142612826		146274826
9	140273252		120143252		138429268
10	135374737		131624737		135413628
11	134452384		131130853		134452384
12	132349534		130303534		132449811
13	114142980		95559980		114142980
14	106368585		88290585		106368585
15	100338915		81341915		100338915
16	88827254		78884754		88827254
17	78774742		77800220		78774742
18	76117153		74656155		76117153
19	63811651		55785651		63811651
20	62435964		59505253		62435964
21	46944323		34171998		46944323
22	49691432		34851332		49554710
X	154913754		151058754		154824264
Y	57772954		25652954		57701691
M	16571			16571			16571
Total   3080436051	2858034764		3076798458

Among these chromosomes, some match but some don't. Could you please
help me explain why? 

Your any attention is appreciated.

Jianying Li