By default, RotaCloud will automatically prorate your staff's holiday allowances based on their start date and final working date. Under the Holiday Allowance column on the Leave page, the asterix next to the employees holiday allowance indicates their allowance has been prorated. Hovering over the figure will display why their allowance has been adjusted.
To disable this feature, go to Settings (the cog icon on the blue navigation bar) and scroll down to the Leave header. Untick the box 'Prorate holiday allowances based on start date and final working date' and your staff's holiday allowance will no longer be adjusted.