This is the condition for the minimum possible vote on hive.vote.
Voting weight * Voting power * HP balance > 15 HP

For example 5% vote at 100% voting power for an account with 1000 HP:
5% * 100% * 1000 HP = 50 HP
50 HP is higher than 15 HP, so it will broadcast the vote at 5% weight.

Another example for 1% vote:
1% * 100% * 1000 HP = 10 HP
Hive.vote will skip this vote because the resulting value is less than 15HP.