Overhead space is reserved for Comfort+ (supposedly) but it doesn't say anywhere that other Comfort+ passengers can't fill it up, causing you to need to check a bag, if you board after Comfort+ boards full. The Comfort+ bins only span the Comfort+ seating section, so they still don't have enough space for all Comfort+ passengers (but this is almost never a problem because Comfort+ passengers still board before regular economy and would simply overflow into the regular economy bins)