Try it with:
LET V_DIFF = Round(Peek('minthisConf', 0, 'min_conf_this_year') -
Peek('minLastConf', 0, 'min_conf_last_year'));
There is no need to transfer the calculation as expression within the further load-statement and to evaluate for each record because it won't change in it. Therefore you could use LET to get the result of this calculation in the variable. Further are the second/third parameter for peek() added because by evaluating a peek-function outside from a load they are mandatory.
If this don't work take a look on the content of your created min-tables (without a drop-statement they would remain within the datamodel - a look within the tableviewer should be enough by just one record) and on the content of your variable, for example with: Trace $(V_DIFF);.
Further if your dates are really dates and no timestamps you could relinquish of the round/floor-functions and for the min-loads you don't need all fields from the database which a SELECT * FROM would query.