Whenever a dropdown is set to allow for multiple values in the field, all values are "searchable" so that you can type in any portion of the selection and it'll limit what you can see to where that string is available somewhere in the values.
Whenever a dropdown is set to require only a single value, the "searchable" opportunity is revoked and you have to type it in exactly as it is listed.
Whenever you have over 15,000 items in a dropdown, this can become cantankerous to keep track of.