Yep this is right. If you have a bunch of candidates polling at 14% then it's very very likely that the polls will be slightly off for one or two of them and they'll wind up getting 15%+.
Also polls have undecided voters so someone polling at 14% probably projects to get like 15-16% of the actual vote anyway. And most delegates are awarded at the district level so being below 15% statewide doesn't mean getting completely shut out.
Notwithstanding all of the above, Nevada uses the 15% viability threshold in individual precincts, like Iowa did, which adds additional uncertainty. In theory it could boost Bernie's margin but Bernie did not get a lot of second-choice support in Iowa.
