In short, I'd say, depth, moisture, paper-layer and biological activity
interact.
I'd differentiate between 'wood chip' and bark mulch. They can have
very different properties. I perfer the waste stream from the arborist
industry which is chipped branches including wood, bark and leaves and
is free as opposed to bark mulch which is relatively biologically inert
(the chemical compisition and structure of bark is designed to resist
fungi and decay and shed water) and provides secondary income to the
industrial logging industry. I have been more satisfied with the more
biologically active mulches at providing weed control.
As a little bit of a sidebar...
I tend to associate wood chip mulch with long term woody plantings. It
seems like woody mulch is much more resistant to self-sowing and plants
adapted meadow-like environments. I tend to mulch deep and once,
knowing that a foot (30cm) of wood chip will become 6" (15cm) of wood
chip in a year and 3" (7cm) in two. And focussing this kind of effort
toward mulching on areas where I want to strongly control composition of
ground vegetation and shift toward woody plantings and forest
understory, or might not have the planting stock to install right away.
I have been reserving patches of a straw topped cardboard over
manure/rockpowder heet mulch where I want more fast growing
self-designing herbaceous communities.. this too may be psychological...
With a DEEP woody mulch, the perennial weeds that survive are often easy
to extract, with most of their reserves spent (easy to harvest dandelion
root!). In a shallow woody mulch then the extra weed control becomes
critical. I tend to focus the paper/cardboard assist at root crowns of
woody plantings where mulch is thin is where . Planting above grade
then helps with avoiding buried root crowns for those species that are
sensitive. I find tap rooted stuff (dandelion, hairy-cat's-ear seems to
pop up again, not necessarily a bad thing) but my rhizomatous grasses,
less so or in thin spots only. I try to stack the odds in my favor and
improve soil by layering manure and rock powder beneath. Moistening the
ground before applying is critical so that weeds are actively growning
and more vulnerable to starvation.
Whoops rambling on again... just been thinking about this stuff lately...