A number of reasons. Many assumptions about how Brexit will go could be factored in to the current exchange rate. If it goes significantly differently to those assumptions - e.g. we end up Remaining, or looking like Remaining - then sterling could rise.
Also, Brexit damages both the UK and EU. So both may fall against the dollar. The assumption is that the pound would fall more than the euro, meaning the pound fell against both the dollar and the euro whereas the euro actually rose against the pound whilst falling against the dollar. But again, as events play out this situation may be reversed.
If the exchange rate you can get is currently 1.17 and you hope for it to be 1.13 for your exchange, tell your currency dealer that and take their advice. But also know what you'd want to do should it hit, say, 1.20 as the date you need to make the exchange approaches. Such an arrangement, specifying a best and worst price for you, might be called a collar. Again, talk to your dealer.