You Don't Like a Good Fit?

K Q J 3 2
K Q 10 7 6 2
------
8 4
10 9 8 7 5 6 4
4 J 8
Q 4 3 K 10 8 7 5
Q 9 5 2 A J 10 7
A
A 9 5 3
A J 9 6 2
K 6 3

SouthWestNorthEast
1 Pass 1 Pass
2 Pass 4 NT Pass
5 Pass 6 All pass

SouthWestNorthEast
1 Pass 1 Pass
3 * Pass 4 * Pass
4 NT Pass 6 All pass* Alerted

Six spades, of course, had no chance, while 6 hearts would have been a cakewalk. Six no fails for the same reason that six spades does, to wit, the uneven break in spades. Yes, had spades broken a benign 4-3, the contract would have finished up the same as six hearts, the 6-card spade holding serving as well as a 10-card, by virtue of top spades -- well, by virtue of top cards in every suit, for 6 no would work on that distribution also. Still, that's little excuse and would still have betokened a poor bidding style even if it worked there. So how did two tables wind up in 6 spades instead of hearts? Well, you see it. Sometimes, oftentimes, one partner falls in love with his suit and won't be deterred by partner. But here North has a stronger heart suit than spade!
In the first sequence, South answers 5 spades to a Blackwood bid. Did North hear a spade suit there? Dunno. In the other sequence, the (alerted) leap to 3 spades was almost certainly a splinter bid, showing a singleton or void, a good fit in hearts and, not so incidentally, the strength to leap to game. I have no idea what that alerted 4 club bid signified. But North could have a far weaker hand than he has for a one heart bid and is only promising 6 points, which, if that were his holding, would make even game a tenuous proposition. On top of which, if you're not sure what your partner will understand by your conventional bid, you'd do well to avoid the convention.
Here, if South is strong enough for a 3 spade splinter, he's strong enough for a leap to four hearts, at which point, with rather solid suits and considerably more strength than thus far promised, we've gotta presume that North would have soldiered on to that heart slam. In any event, it's self-evident that the more balanced 10-card fit would have served very well here and should have been discovered.