It could be that she's trying to protect you from something she thinks will stress you out. Or perhaps she's not sure how to explain the full situation clearly.
There could be various reasons. Maybe she's embarrassed about some part of it, or she's still processing it herself and not ready to share everything. Another possibility is that she doesn't want to burden you with all the details.
Maybe she's afraid of your reaction or worried it might cause problems in your relationship.