The following is from Hewlett-Packard documentation, http://www.hp.com/cposupport/printers/support_doc/bpl02119.html
All HP LaserJet 4/5/6 family printers have a page count feature built into the firmware. However, this feature works differently depending on which HP LaserJet printer is being used. The following is a description of how the page count feature works for each printer within the HP LaserJet 4/5/6 printer families.
HP LaserJet 4/4M printers
HP LaserJet 4 Plus/4M Plus printers
HP LaserJet 4P/4MP printers
HP LaserJet 4Si/4Si MX printers
HP LaserJet 4ML printers
HP LaserJet 5P/5MP printers
HP LaserJet 6P/6MP printers
All of the above printers use the same method for keeping track of the number of copies. There are really two different page count values: Primary and Secondary values. Every time a page is printed, whether it is an internal job (such as a self-test) or a standard print job, the Secondary page count increases by one. This value is stored in standard RAM. Once the Secondary page count value reaches 10, the Primary page count will increase by 10. The Primary page count value is stored in a type of memory called NVRAM (Non-Volatile RAM). This is important, since NVRAM is not cleared when the printer is powered off. Standard RAM, on the other hand, is cleared when the printer is turned off or reset. Thus, the Primary page count only increases in increments of 10.
Example
You have a brand new HP LaserJet 6P printer and you print a self-test page. When you look on the test page for the Page Count value, you will see that it says 1. Next, you decide to print a two page letter and, after that, another self-test. The page count value now says 4. Internally, the printers Secondary page count (stored in RAM) has the value of 4 while the Primary page count (stored in NVRAM) still has the value of 0. Now, you turn the printer off, then back on, and print another self-test. The page count value again says 1 since the previous value of 4, stored in RAM, was cleared when the printer was powered off. Finally, print a ten page document and then turn the printer off. Upon turning the printer back on and printing out another self test, you see that the page count value is 11. Internally, the Secondary page count value is back at 1 while the Primary page count value (stored in NVRAM) is 10. Added together, you end up with the resulting value seen on the self-test page.
The HP LaserJet 4L/5L/6L printers differ from that of the other printers in that they do not have any NVRAM available for storing page count values. Thus, no way exists for the printer to retain a page count value once the printer is powered off. The HP LaserJet 4L/5L/6L printers have only a single page count value that increases in increments of one until the printer is powered off.