Why does the threshold apply to the MUST_SIGNAL phase in revised BIP 8 (lot=true)? Why doesn't it enforce 100 percent signaling?

Question

BIP 8 states that:

During the MUST_SIGNAL phase, if (2016 - threshold) blocks in the retarget period have already failed to signal, any further blocks that fail to signal are invalid.

Why doesn't it enforce 100 percent signaling during the MUST_SIGNAL phase rather than merely the threshold?

Answer 1

nickler answered this on IRC.

In a mixed network, the rules can become active, but in two separate chains because lot=true nodes will reject blocks from the lot=false chain which doesn't require 100% signaling.

The MUST_SIGNAL phase (when lot=true) overlaps with the last two weeks (2016 blocks) of the STARTED phase (when lot=false). In the case that the threshold of miners signaling is met (but not 100 percent of miners signaling) during these two weeks you don't want to create a chain split between lot=true and lot=false nodes for no reason. If MUST_SIGNAL enforced 100 percent signaling and there wasn't 100 percent signaling during this period you would get a chain split between lot=true and lot=false nodes.