What is difference between OGC and ISO standards - why do some open specs carry both standards?

Practical difference from users point of view is that you must pay for ISO standards but OGC standards are available for free. See for example https://www.iso.org/standard/40114.html if you want to pay 158 Swiss francs for Simple feature access — Part 1: Common architecture.

ISO and OGC are different and independent organizations but they co-operate. OGC is somewhat more agile and it can get standards accepted faster. ISO has more official status and especially actors on public and defense sector often require that just ISO standards are followed.