Tiger -- what gives?

[Skehan Adrian]

I downloaded the 10.4.1 update today and installed it on a couple of  
800 Mhz eMacs and 800 Mhz iMacs with on problems, Tried the same on  
an iMac 1Ghz., an eMac 1 Ghz. and iBook 900 Mhz. and it would not  
install because "This volume does not meet the requirements for this  
update", this strikes me as a bit unusual!  Does anyone have an idea/ 
explanation why older machines accept the update and more recent  
machines (less that 2 years old) wont? and does this mean that  
updates will be model specific for Tiger?