Emil, how do you measure the solutions? (a.) Drop count (by pipette) or (b.) proper ml (by syringe)? If the former - (a.) that is... - mind that different solutions' specific gravity and surface tension are also different, therefore the drop volumes are different - hence the different rate of consumption... If the latter, I can't think of anything other than the phenomenon may be caused due to different vapor pressures (highly affects evaporation speed / rate) of the solutions. (But, most probably that can be omitted if you aren't using very large containers / small amounts of solutions and I highly suspect it would cause a significant difference...)