Budget for one, the Army has a certain number of 4 year and 3 year scholarships, they have a finite amount of money for scholarships. By giving a mix of 4 and 3 year scholarships they can reach more cadets.
Tuition cost for the school can have an effect as well.
School selection can also come in to play, if the school your daughter selected had a large number of applicants and was able to fill their quota could be a reason. The school only has a certain number of scholarship slots available, a mix of 3 and 4 year. Some schools don't fill up as fast or have a smaller number of applicants that list the school.
The application itself can also be a factor, CC may want to see how a cadet performs their first year. To be honest there are more 4 year scholarship winners that don't make it through the first year then you might think. When my younger son started there were three 4 year scholarship winners, my son was the only one that passed the APFT, one dropped the program after 3 months and one left at the end of the year. The Army wasted the slots on these cadets, they ended up giving scholarships to two other cadets the next year in his class.
Giving more 3 year scholarships have given the cadre at the battalion level a chance to evaluate cadets, some will receive upgrades to their scholarships if the funds are available.
And of course there's the fact that nobody really knows how they decide who gets a 4 or 3 year scholarship, pulling names out of a hat has been one theory, even darts have come up a couple times.