Weird slider behaviour

For any use of a slider in my organisation we need to add keyboard controls for accessibility purposes. To do this I add the following triggers for each slider:

Add value 1 to Slider1
When the user presses RIGHT after clicking on this slide
if Slider1 is between 0 and 5

and

Subtract value 1 to Slider1
When the user presses LEFT after clicking on this slide
if Slider1 is between 0 and 5

The weird behaviour has happened when a user uses their mouse to drag the slider and then uses their keyboard arrows. When the user does this the slider begins to move by 2 steps at a time instead of 1. When trying to rectify this issue the slider then began jumping steps as described, but without first using their mouse to control the slider.

Has anyone experienced this issue? If so how did you rectify it?

P.S. As a side note it would be helpful if there could be default keyboard shortcuts for sliders and dials.