It may be something totally different in your case, but the same thing happened to me last week where a refund of ~200 was supposed to be returned, and initially I got 82.5. However, about five days later [today] the rest was credited back in a separate payment. I can't explain WHY this happens, but sometimes the refunds are split maybe if payments were sent separately or with echeck or were on hold temporarily. I asked customer service via chat for an exact date by which the remainder should be sent to my bank account, along with a screenshot of the confirmed dispute, and they told me exactly when the rest would be available. Hopefully this helps a little!
