No one thinks Halo flopped.
Even if the actual sales are only half of what Halo 3 did.... it wont matter.
Halo is so big it can lose half its sales rate and still be a monster sales wise.
But yes, from what we know of sales numbers (only from the UK so far), it looks like it sold half of what a normal mainline halo game does.
Sony side = halo is under preforming (sales wise). UK sales are half what it used to be.
MS side = it generated 400million in revenue, who cares how much it sold, it made bank!