Eh?
If you are looking for a player "who will lift the team to a greater level" there would be no players who are "just about good enough bulk out the squad as cover for a match or two - not to be relied on to play a lot of games, but for a game or two can just about do a job". Those players would not have made the list. The list would consist of players who could all lift the team to a greater level, all of them. Obviously this is in the opinion of the manager and recruitment team.
Not necessarily. When you have one player in a specific position, let's say left back. You have one. You need someone else. Since the one you have was signed to get you out of league one into the championship but has been exposed a few times you want someone better, or at least a second choice.
Your list will (for illustration's point) contain someone who is 4 times better, he is the one you most want, your second choice is a player who is three times better, third choice is twice as good.
Your last choice might be one who is not quite as good as your current guy, but it gives you that squad depth you need, so that the guy who you already have can be rested for a match, and not make as many mistakes or so that he sees there is someone else at the club who could take his place forcing him to focus more in the hopes of avoiding losing his place, or just so you have someone to cover that position should he get injured (such as Lita or Obika provided with the striker position last season).
Ideally, every signing improves the team, but it doesn't work out that way. Sometimes you have to settle for the minimum, improvement of squad depth. That's not the worst thing ever, but that is why getting the last choice on the list isn't as good as getting the first choice.