Voltage drop could be a factor, at least for some panels.
For example, take a look at the spec sheet for the Kyocera KD145 SX-UFU: https://www.solarelectricsupply.com/fileuploader/download/download/?d=0&file=custom%2Fupload%2FKyocera-KD145-SX-UFU-solar-panel-datasheet.pdf
The listed voltages...
@Mimisah, it appears that you them wired up in parallel, if you wire them up in series, you should see the voltage double.
https://www.google.com/search?q=solar+series+parallel