It’s the difference between an authorisation and then the later presentment/settlement.
What happens immediately is an authorisation, Monzo is instructed to hold that amount of money aside and make it unavailable for spending elsewhere before a later presentment file comes in to actually ask for the money to be transferred during settlement.
What happened in your case is the authorisation happened but not the presentment (normally I see this happen on the night that the product ships or on the night of delivery day), your “refund” is actually either canceling that authorisation or simply letting it expire. In the latter case, Monzo support is capable of canceling the authorisation before it expires, enabling you to regain access to the held funds earlier.
You’ll see in other banks there may be two numbers, the “true” balance as of last settlement and available funds, the former does not take in to account any authorisations while the latter is the one actually used for keeping track of the account balance during the day. Monzo uses available funds as it is the more accurate to what you have avalible to spend.
Some more information on that flow towards the middle of this post.