Here's how it works, if you just play it straight:
- Get all coins in the open
- Get all coins in the ?-bricks
- Pump one brick that you can hit repeatedly for about 15 coins
- Go down two pipes and get all coins in those bonus rooms
- Either have the fireball or be small so you can get coins in low tunnels at the end
- Jump from the highest platform at the very end to get all the coins near the top of the screen
So you revisit the brick with all the 15 coins in it and perfect your timing to get 15...16...17 coins. This appears to be the true limit on that one brick, but you're still only at 98 coins.
Finally, you remember that there is one ?-brick just to the right of each of the two pipes that lead to bonus rooms. Just like what you see here:
So you first go past the pipe, pump the ?-brick, then go through the pipe, and when you return you can pump the ?-brick a second time. Do this for both pipes and you'll have the final two coins for an even 100, and an extra man to boot.
This has to be intentional. It's like the designer leaving a hidden message for the devoted player: "You thought it through and did it perfectly. Good job."
Is it intentional? I don't know, but probably not. It just looks like something odd that seems a little strange, but in the end is probably just coicidence, like the "minus world" in Super Mario Bros, which was actually a bug.
Mario games have traditionally been fairly loose with extra lives. For example, if memory serves there are two extra life hearts just lying around, one in a block and another in an invisible block.
Coins, which are essentially a handful of points and a hundredth of an extra life, make great "snacks," bonuses that feel good to collect but don't actually help the player *that* much. There are areas in that very game you're playing that are full of coins to the point of absurdity, enough so that I doubt there's significance to a precise number of collectables in the first level.