It's the base success part that throws that off.  I'm assuming the errata stats here: 

Base success (0 degrees) HB: 1 hit SB: 2 hits

1 degree HB: 2 SB 2

2 degrees HB: 3 SB 4

3 degrees HB: 4 SB 4

4 degrees HB: 5 SB 6

5 degrees HB: 6(MAX) SB 6

6 degrees HB: 6 SB 8(MAX)

Nathiel said:

This table is accurate- it takes 5 full degrees of success to get max hits with the HB and 6 with the SB because of the initial success to hit. 

Raw damage output would have the SB doing somehwere in the neighborhood of 10-12 more damage on a 'full hit.' 

But I'd say in response to that there are a lot of other factors that make up the weapons. 

1. Damage Output: The HB's individual shots will, on average, do 3 more damage and 1more pen than the SB.  As a GM I can say extra pen and slight extra damage per hit can be of substantial importance.

2. Range: The Heavy Bolter fires 50 meters further than the storm bolter using standard ammunition

3. Cost: Both Weapons are 20 req

4. Renown: HB is available to all marines, the Storm only to those of Respected or higher (which means you have to play a bit before you pick one up)

5. Clip Size: By default, the Heavy Bolter comes with a backpack of 250 rounds, otherwise known as 41 ridiculous rounds of continuous firing before needing to be reloaded versus the Storm Bolter's 60 round clip which will go in 7 rounds.  Not important in all campaigns I know, but it's an item of note.

6. Enemy Dodges: An enemy dodges one hit per degree of success against a heavy bolter, but two hits per degree of success against a storm bolter

All in all, I think they're two different weapons, used by different folks for different reasons.  In some cases I still agree with DJ though about some of the weapons feeling 'better' but for me that often comes down to specialty ammo.  The Boltgun was made an excellend all arounder, and sometimes other weapons feel so terribly specialized it can be annoying and feel like it's counter to the existing fiction.