Because a belcher only has 3 attack for ~71% of it's life so it is overvaluing it to say that it is a 3/7.
Likewise it's not fair to say highmane is a 6/9 = 15 stats. I would rate it as:
5 + (5/9) * 6 + 2 + (2/9) * 2 + 2 + (2/9) * 2 = 13.2 stats or a bit better than a 6/7.
How much better than a 6/7 is going to depend on how much you value the card needing 2 damage sources to kill. I think that is worth about the same as other unique abilities which is why I would say haunted creeper at 5 + ability is equal to a knife juggler who is also 5 + ability.
belcher is 10.4 + ability which is better than, say, spectral knight at 10 + ability
highmane is 13.2 + ability + beast which is crazy strong. Probably even better than fire elemental.
edit:
"Requiring two damage sources" is a good ability to have. But in general split stats are not better. This can be shown from the fact that no one plays silver hand knight in constructed, but if there was a 5 mana 6/6 everyone would be playing it because it would be grossly overpowered. So any good evaluation of card stats should show that silver hand knight as a 4/4 and a 2/2 is worse than a 6/6.