No I don't think the display is playing up. Use a wattmeter to empty your battery and determine its capacity. However, this will only give you the weakest capacity element (for instance, if you get 8 Ah for 10Ah elements, that means the other elements have between 8 and 10 Ah). It's normal that the voltage diminishes when the battery is under effort, especially when it ages.
If your battery gives you satisfaction otherwise, don't bother and continue using it like before.