An alliance can have 100 members/allies, yet the embassy can only hold 10 of these allies troops at a level 10 embassy. so if everyone in an alliance of 100 members sent troops to reinforce a level 10 embassy then only 10 of the allies troops would be housed, the rest being returned. hardly intuitive now is it?
Also, since each Embassy level/spot allows only one allies troops then that spot would be best filled with a large number of troops rather than an ally sending a small number. i.e. player 1 might fill the spot (reinforce) with 25k troops which is inefficient when player 2 can fill the spot with 100k troops. the defender would likely choose to send home the smaller troopcount so their embassy can accomodate a larger number of troops from another ally, in which time they could have been attacked. The Embassy is a very poorly thought out function of this game considering the number of allies/members in an alliance. a better way might be to have a maximum number of troops housed per embassy level. or better still have no limit. if the defender cannot sustain & feed the reinforced troops they have the option to send home (or get the reinforcers to send more food to support the amount of troops they've sent).