Because it caches all the objects and calculation so it does not have to do that again next time you open the sheet.
This is default functionality that can not be changed and is there for your joy. If it would not do this and you have heavy calculations, it has to do these every time you re-open the sheet.
When ever there are any calculations done, the results are saved in the cache to improve performance. When any other user using the same dashboard looks at the same details as you did, it directly reads it from the cache instead of recalculating it over again. The cache is cleared on a regular time basis. If the memory hits the brim, the cache gets cleared as well. This could be a possible reason why its happening.