Probably either a printing error, or having some of the powder undissolved when you make up to volume doesn't matter. I'm going for the latter; the reason to mix in a smaller amount and then make up to volume is because the volume increase due to solutes isn't easy to calculate (and depends to some extent on the water source/purity); as long as you don't expect an exact volume after mixing, and make up to the final figure, starting with a lot less won't do any harm (though you might be well advised to make up the final volume in steps when headed from 800 ml to 5 L).
Then again, the only things in D-76 or Mytol that require a lot of water are the sulfite and borax -- and Adox has eliminated the borax/borates, so it might well be that their 5L packet will fully dissolve in 800 ml of water. If so, storing it as 1L ought to extend its solution shelf life by a large margin (don't forget to dilute 1+4 before calculating other dilutions or replenishing).