If the number of Bitcoins in the UTXO is always equal to the number of mined Bitcoins
This is an apple : oranges comparison. The size "Number of Unspent Transaction Outputs" set is independent from the number of bitcoins in existence.
An unspent transaction output might be for a fraction of a bitcoin (some number of satoshis) yet it still counts as one unspent transaction output, a member of a very large set (over 55 million UTXO's according to the chart provided).
To put this in other terms, suppose every single UTXO was spent into a single bitcoin address. (It will never happen, this just a thought experiment) Then that address would hold all the bitcoins in existence, (a bit more than 17,100,000) and then there would only be one member in the "Unspent Transaction Output" set.