The mass of an empty beaker is 89.90 grams and when the beaker is comined with stoppers the mass becomes 123.44 grams. This means that the mass of the stoppers alone is 33.54 grams. However, when stoppers are direcly placed in the balance the mass of the stoppers is 33.59. Why are the masses of the stoppers a bit different from each other?

The mass of the empty beaker 89.90 grams The combined mass of the beaker + stoppers 123.44 grams - The mass of the stoppers only 33.54 grams