There are two separate things. First, StakeDrop was partially send on Ethereum, but there were still many people not claimed before the Khala launch. So when Khala launched, we distributed all the remaining reward to their KSM address. The second is that, no matter where the StakeDrop is distributed, the funds were not "locked" on Ethereum anymore. So where it lives in, Ethereum or Khala, it MAY be bridged to Khala. So the full amount should be minted on Khala (as PHA in circulation for our final reward distribution, and PHA in reservation for the part distributed on Ethereum)
In the initial tokenomic paper, there's 63M reserved for crowdloan and 27M reserved for StakeDrop. However in the reality, only 21M is used for StakeDrop, leaving 6M. We decided to use that for Crowdloan.
Here's the problem. When we initially set up the bridge, we mistakenly thought the bridge reservation for the corresponding part should be (21M stakedrop + 63M crowdloan). But actually it should be (21M stakedrop + 69M crowdloan). That's why there's 6M missing.
Finally, the bridge reservation on Khala is a way for protection. Suppose the bridge is 100% secure, we can safely put 1B in the bridge reservation. That will be sufficient for all the future birdge liquidity. Or we can just adopt the "mint & burn" approach without caring about the reservation.
A precise reservation amount is the best, because even if the bridge is compromised, there's a upper limit of the damage. A less reservation may cause trouble in extreme situation, but there's no immediate damage.
The timeline is StakeDrop > Khala Crowdloan > Win > Distribute reward on Khala > Enable bridge