For a player who is at risk of not making the NHL, and thus at risk of not receiving their full salary, then signing bonuses are more beneficial, because they are guaranteed that money regardless of where they play. This has no relevance to a player like Marner, who is guaranteed to be in the NHL and get his full salary regardless.
In a trade scenario, Marner would be worth more and the Leafs would have more options, as the actual salary cost of Marner for a receiving team would be less than his cap hit. This doesn't mean the intention is to trade, but it's always good to utilize any available options to increase the value of your assets.
The cap hit is probably lower than they expected, but that's because they likely expected well above market on an offer sheet, and they expected to sign Aho to an 8 year deal, not because 8.5m x 5 is below market value.