What the returned bundle is
The mark is not printed onto your sheet. It arrives as its own slip, attached to the paper it describes and naming the official whose signature it confirms. From that point the two are a single item. Handle them as one, because a slip alone says nothing about any paper, and a paper alone has lost the proof that came with it.
Why the bundle has further to go
Guinea-Bissau has not joined the Hague Apostille Convention. Outside that agreement a British mark does not carry on its own, so document legalisation for Guinea-Bissau means passing the finished bundle to consular staff acting here for a second mark. They work from what is already attached to the sheet, which is why the bundle has to reach them whole.
That second stage is also why the file is away from you longer than on a treaty route. An apostille from United Kingdom to Guinea-Bissau makes two journeys before anyone abroad reads it, and it waits in two queues rather than one. Neither wait is shortened by anything you can do at this end once the envelope has gone.
Keeping several papers straight
Where a few papers go at once, each returns with a slip of its own. The sensible thing is to keep them paired rather than sorting the lot into a stack of sheets and a stack of slips. Anyone reading the file later reads it as pairs. A mismatched set is the sort of small mess that costs a week for no good reason at all.
Two things to settle first
Two decisions shape most of this route. Choose the sheet you could replace if it went astray, since more than one office will handle it before it comes to rest. And count the papers honestly, because the charge lands on each document rather than on the envelope that carries the whole lot.