Exchanges typically determine the price of an asset from their order book which is a listing of offers to buy/sell an asset in varying amounts and prices.
Most cryptocurrencies rely on private exchanges to provide a market for cryptocurrencies, but the XRP Ledger has a Decentralized Exchange (DEX) built into the XRP protocol.
Some exchanges post the value of XRP from their private exchanges.
The XRP exchange rate shown in XUMM is fetched from the DEX on the XRP ledger.
XUMM only offers interaction with the XRP Ledger.
Because the different order books (e.g. private exchange book vs. XRP Ledger Decentralized Exchange) contain different sets of offers the price will diverge somewhat. This is usually temporary and the values will tend to converge and diverge over time.
This difference is normal and affected by things like the total number of orders in each book, the velocity of the transactions (how fast each exchange completes transactions) and buyers/sellers perception of the value of a given asset at any given time.
The difference can also come from logistical factors like time zones. E.g. if an exchange is in an earlier time zone the order book will reflect events that transpired while the exchange was closed earlier than one in a later time zone.
This difference can be advantageous or disadvantageous depending on the value of the asset held at time of purchase and can be reconciled by arbitrage - buying in one market and selling in another to take advantage of differing prices for the same asset.