Lurker HP research:
Addon that calculates the HP. Addons are not that accurate but it is so close to 3.8, no where near 5.3. This fight is done late March/Early april as seen from the upload date (http://damoc.free.fr/index.php?rep=videos/)
http://damoc.free.fr/videos/DnT_Lurker_High.mp4
4:15
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Recount:
Nihilum world first video on March 14th 2007:
https://youtu.be/eZJNTmHA7Cg?t=109

You can see Lurker at 75% HP here, and damage done on the top right. No add damage is added since this is all damage done to Lurker so far.  We only see 1-8 damage dealers, but I will stimulate 9-15 as well, implying they had 15 dps excluding tank damage.
1. 82777
2. 82041
3. 78476
4. 77518
5. 76853
6. 64337
7. 57197
8. 55040
9. 53000
10.51000
11. 49000
12. 48000
13. 47000
14. 45000
15. 44000
= 911.239 x 4 = 3,644,956
Ofcourse I am stimulating the last 7 DPSers and keeping in mind that addons are not 100% accurate. It still comes very close to 3.8 million HP than 5.3 million ofcourse.

Last resort on March 18th 2007:
https://youtu.be/BkdRePHzjY0?t=108
20% damage done against Lurker. Again I will simulate the rest of the DPS.
1. 66653
2. 63094
3. 61014
4. 54465
5. 52151
6. 51030
7. 48873
8. 45787
9. 44000
10.43000
11. 41000
12. 40000
13. 39000
14. 37000
15. 36000
= 723.067 x 5 = 3.615.335

Again, close to the 3.8 mil, not even close to 5.3 mil HP. This kill is also DURING the period of the 5.3 mil at wowwiki.
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My theory about it being 5.3 is that people just counted total damage done the whole fight and it came to about 5.3 cause ofc the add damage is also calculated.
Most fights I have watched had 4 add phases so I will calculate that damage now.
(Coilfang Ambusher)27.000x6 + (Coilfang Guardian)70.000x3 = 372.000 x 4 = 1.488.000

So you got 3.8 million lurker HP + 1.5 million add phase damage = 5.3 million damage done WHOLE FIGHT

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Wowwiki:
March 16 - April 23 2007 | 5,311,600 million HP: http://wowwiki.wikia.com/wiki/The_Lurker_Below?direction=next&oldid=602638
April 23 - June 22 2007 | 3,511,600 : http://wowwiki.wikia.com/wiki/The_Lurker_Below?direction=next&oldid=603666
It seem very weird for me that literally 3 and 5 just switched positions. This is only my opinion, but I would expect 6 of those digits would be different with an HP change, not just moving the position of 2 digits.

June 22 2007 - November 8 2008 | 3,800,00HP: http://wowwiki.wikia.com/wiki/The_Lurker_Below?direction=next&oldid=704936

Why the change from 3.5 to 3.8? Maybe Blizzard buffed the HP by a couple of hundred  thousand randomly between the above date, perhaps people got a more accurate read of the HP as more guilds did it and addons got updated. I think the latter.

November 8 2008 - 2016: 2,655,000HP
Either someone trolling HP, or Blizzard nerfed HP with the release of WOTLK. I have no clue, but this should not matter as its pretty much WOTLK+

(April first 2007 )Comment about 5.3 mil HP VERY likely he just read if off wowwiki which was wrong imo. Putting it here just in case: http://www.ownedcore.com/forums/world-of-warcraft/world-of-warcraft-guides/15193-shade-of-aran-guide.html#post120561