They can't.

iLvls 26-40 refer to one of three socket tiers, specified for each item type.
All spears share the socket triple {3,4,6}. That is, max 3 at iLvls 1-25, max 4 at iLvls 26-40 and max 6 otherwise (iLvl > 40).

However, in order to determine the maximum sockets at a given iLvl, one also needs to know the maximum sockets allowed in the base item (e.g. Ghost Spear, War Pike).
This is the number displayed on the Weapons and Armor pages of the Arreat Summit website.

The maximum number of sockets possible at a given iLvl is then the minimum of the two socket figures (the iLvl-dependent one relating to item type, and the one relating to the base item regardless of iLvl).

Socketed items dropped from monsters or terrain objects, or available in town merchant inventory, cannot exceed 3 sockets in Normal difficulty and no more than 4 sockets in Nightmare.

This post was edited by Eywa on Oct 27 2012 11:34am