It probably has no real significance. Manufacturers often update part numbers simply because of (for example) a change in packaging.
CartridgeSave (who're a well-respected supplier) use both part numbers here:
https://www.cartridgesave.co.uk/CH561EE.html
If you're still in doubt though, use the numbers that appear on HP's own website:
Black: CH561EE
Tri-colour: CH562EE
Black High Yield: CH563EE
Tri-Colour High Yield: CH564EE
http://store.hp.com/UKStore/Merch/Supplies.aspx#!q=HP%20301%20Ink&type=cartridge