If they do announce today, I assume it will be in the evening and I doubt it makes a difference if they announce today or Friday. So might as well let the news out today!
As for current spots, I think they are trying to fill in the possible international student seats which some may not like the online because it will be night time for some of them. The whole experience of being at Cambridge for intl students will be weighted much heavily and may apply again next year. Also, even if they have to fill 200 seats, they probably have to take a chance to admit 250 or 300 for yield and some may not to commit due to low financial aid and other reasons.