The difference comes from comparing the May forecast submitted on theMay 19 May, 2020, which is based on assumptions and estimates for the month of May, and the May invoice submitted on 19 June 19, 2020, based on actual costs.

The forecast is updated with the actuals once those become available. Therefore, to get a more accurate view, the invoice for the month of May should be compared with the forecast submitted in June, July, or later (once the May actuals are available). The revised variance then becomes c£37k (£213k forecast vs £176k invoice).

The key reasons for variances are:
The forecast is based on accruals accounting, and the invoice is based on cash – this means that costs such as council tax for the office, rent, and rates for both warehouse and office would be invoiced based on actuals quarterly but accrued monthly meaning that in the month where we invoice you the quarterly rent, the invoice would be higher than forecast, then. Then, on other months, variance would be other way around. The same applies to training and similar expenses – the costs are accrued the month it has taken place but usually invoiced a month later due to time it takes to pay the supplier and pass costs to SU.
In the forecast, personnel is accrued at 100% rates vs invoiced at 88% plus bi-annual milestone invoice followed after the KPI scores are accounted. In the example of May, this creates c£20k variance.