Why is the Bill Amount automatically updated with a Write-On Event?

I am testing with write-on/write-off events. I am using a functional currency of SEK, and creating revenue and invoices in EUR. In my test case, I created an invoice for EUR 200,000 and a functional SEK invoice amount of 1,760,260. I then created revenue over subsequent months. The revenue totals EUR 200,000 and the functional revenue totals 1,760,095.33. Therefore, I need more SEK revenue to match my original SEK invoice. I presume this means that I need to write on some revenue -- an amount of SEK 164.67.

When I try to create a write-on event, I only want to adjust the revenue in SEK. The transactional EUR amounts are correct and do not need to be adjusted. However, when I create the write-on event, the bill amount is automatically updated with the amount that I enter in the revenue field. Why does it do this? I am not intending to bill the customer for the FX impact. The whole reason they require billing in EUR (or some other currency) is so that would bear the risk of the FX impact.