what is the difference between mindfulness meditations and focusing on other things?
It is question I've been asking myself as well in my experience with mindfulness meditation.

Meditation is not manipulating experience, it is simple noticing it. Furthermore it focuses on sensory experience which seems to be inversely correlated with our narrative experience(Farb et al., 2007).

Being focused on studying involves planning and invariably it adds a narrative feature to our experience. Mindfulness increase grey matter in areas associated with executive control(Hölzel et al., 2011), hence we can hypothesize mindfulness is good for more effective studying. 

There might some type of false placebo effect in studies reporting increased focus due to meditation.

Farb, N. A. S., Segal, Z. V., Mayberg, H., Bean, J., McKeon, D., Fatima, Z., &amp; Anderson, A. K. (2007). Attending to the present: Mindfulness meditation reveals distinct neural modes of self-reference. Social Cognitive and Affective Neuroscience, 2(4), 313–322. https://doi.org/10.1093/scan/nsm030

Hölzel, B. K., Carmody, J., Vangel, M., Congleton, C., Yerramsetti, S. M., Gard, T., &amp; Lazar, S. W. (2011). Mindfulness practice leads to increases in regional brain gray matter density. Psychiatry Research, 191(1), 36–43. https://doi.org/10.1016/j.pscychresns.2010.08.006