This issue is most likely due to an "extra charge" configured on the Upsell. The "extra charge" is in addition to the price of the Upsell itself. The price of the Upsell is the base amount while the extra charge is really intended for when you plan to sell multiple units of an Upsell at once or have another Upsell option configured that would warrant an additional charge on the unit (i.e. ordering an XXL Shirt).

Example Scenario
If you were configuring an Upsell for a commemorative towel you would set the value of 1 towel to $20. Then, you could add the option to add multiple towels to a registration order. For the quantity of 2, you would configure an "extra charge" of $20 to bring the total sum to $40 if a participant decides to buy 2 commemorative towels.

Configuring an Upsell with an extra charge