Faint positive on 25mlu & hardly visible on 20mlu brand?

Why did I get a faint positive on a 25mIU/ml test? But hardly visible on a 20? (Different brands) so confused? Surly it would of been more visible on the 20?

(One with the red line is 25mIU/ml) this was taken last night and the 20mlU was taken this morning :/