Unit of analysis issues. The level at which randomization occurred in each of the RCTs was taken into account so that the number of observations coincided with the number of randomized units. If we included clinical trials with multiple arms and one arm was considered more than once in the same comparisons (e.g., different dosing schedules of the same therapeutic agent compared to the same control group), we combined all relevant treatment arms in a single group and compared with the control to avoid double counting of participants in control groups.
The statistical method usually used to combine the results of multiple studies is to weight them by the amount of information they provide (more specifically, by the inverse variances of their effect estimates). Those trials that contribute more weight are mentioned.