The differences can be attributed to the device used. The air within the syringe escaped slowly, due to slow air leaks. The differences can be again attributed to the syringe. As mass increases, volume decreases, and vica-verca. With increased pressure and nowhere for the air to go, the air is forced to occupy a smaller area, decreasing it's volume.
Force is found by multiplying Mass by gravity, and gravity is a constant.
Because of the increased weight being placed upon the syringe, or other object, there is greater pressure. As the volume descreases, pressure increases, and vica-verca. According to the paragraphs above, with rising force Volume decreases and pressure increases. If you want proof, look at the data tables . This lab was doomed from the start.