The short answer is because of the fluctuations in the exchange rate between US or Canadian dollars and Mexcian pesos. You will normally pay all of your closing costs and acquisition tax in pesos, but the exchange rate between MXN and USD or CAD is notorious for its occasional significant fluctuations. Any changes in the exchange rate that occur between the time that you sign your Contract to Purchase and the date of closing have to be factored into the final closing costs.

Let's say your closing costs in pesos when you signed the Contract to Purchase were estimated at $510,000 MXN. If the exchange rate was 17 to 1 at the time, this would equal a total of $30,000 USD.  

But, if by closing or even a couple of days before closing the exchange rate jumped to 16.5 to 1, the dollar amount would have also jumped to $30,909, costing you $909 USD more than you had originally calculated. Of course, this same dynamic can work dramatically in your favor as well. If the exchange rate went to 17.5, then your closing costs would be $857 USD less than you had originally calculated. 

It's for this reason that we normally ask buyers to send an extra $1,000 or more, depending on the purchase price, to cover any changes in the exchange rate. The money that is not necessary will be wire transferred back to your bank account.