The difference is probably the same as with the evangelical Born Again Christians being far more fervent and 'cultishly inclined' than your regular Joe Shmo adherent who is just a regular guy who has always gone to church, but practically indistinguishable from the neighbour who doesn't, save for a slightly different social scene on Sundays.
Of course there's also probably equivalents to the more cultish Christian communities, but they tend to keep to themselves, and don't tend to relocate en-mass into the heathen lands (missionaries aside) and make their presence more obvious than than some would like.
The post-resettlement rebound into fervency by the offspring also depends on the availability of cultural reappropriation. Hard to compare with historic waves. The Pilgrim Fathers were self-exiling themselves and their beliefs, as a badly summarised example. And even in cases of colonisations that had not brought very definite views with them, their children and children's children would not have had the same near-instantaneous communications from the Old Country pressures to reconform to any baser belief from which the more open parent-settlers had drifted, out of inclination, inspiration and/or necessiry.
It's not an Islamic thing. It may be more obviously one due to the relative youth of Islam, in cultural terms (Christianity has largely gotten over its prosthelytising phase, Judaism doesn't really 'do' recruitment at all (Kabbalah aside), I don't know too much about Hinduism/Sikhism/etc, and there's issues with conflicting cultures there of course, but one can probably put them in the 'ancient and stable' camp. Buddhism and other popular targets for New Age spiritualities has its stable and ancient core and the potentially 'problem child' offshoots into conflicting cultures, but being mirrored by the contemporary Christian missionary excursions into China, and especially the House Churches (protestant) and Underground Churches (catholic) as separate from the state-sanctioned portals for each separate belief (protestantism and catholicism being considered very different from each other, over there, I understand, though not to sectarian levels).