OpenGL Framebuffer/FBO RTT subpixel movement discrepancy

Question

My program uses orthpgraphic projection with an internal resolution of 480x270 and a viewport resolution of 1920x1080 (without shaders). When I draw a rectangle directly to the framebuffer I am able to move it around in incriments of 0.25 pixels (because there are 4 pixels in the framebuffer for every 1 pixel of internal resolution). But when I switch from drawing directly to the framebuffer to redering to a 1920x1080 FBO texture, instead of the 0.25 pixel movements, it "snaps" and only moves 1 whole pixel at a time. Why is this?