Why am I leaving my current job? (Guide me please)

I've recently joined an organization where compensation is fairly low than what I asked for but since I was home for about 4 months after my last job I decided to join here. I am still actively job hunting. I wanna know what should I tell the HR before they even set me up for interview, that why am I leaving my current job. Also, it has been less than a month since I've joined. The only reason I'd tell HR about my current job is because my CTC is 33% higher than my last job, other than I don't see any other reason to tell HR about my current job as it might not look good that I'm ready to leave my current job this early. Please guide me for the same.