It is the actual earnings paid* (or for self-employment - gross profits made) in each of the claimant's Monthly Assessment Periods (MAPs) that the DWP will use when assessing the claimant's entitlement to Universal Credit.
Any earnings from employment or self-employment, above the claimant's work allowance will reduce the claimant's Universal Credit award by 63% of the amount above the work allowance.
This means that as earnings fluctuate, the claimant's Universal Credit award will also fluctuate.
*However a High Court judgement published 11/01/2019 ruled that it can sometimes be the MAP in respect of which
the earnings are paid that determines for which MAP they are attributable. In the four cases under consideration it was ruled that where two payments were made in the same MAP they should be treated as paid in the MAPs for which they were intended.
The DWP have an online Employers' Guide to UC: here