5 is 4 + 1. The injection of consciousness (1) to matter (4).
7 is 4 + 3. The injection of spirit (3) to matter (4).
Both 5 and 7 represents creativity. 5 feels and seems more mental than 7. 7 is more about emotions and being inspired.
I also wonder about 7 = 5 + 2. The combination of quintessence (5) and polarity (2). It’s the polarity which perhaps “lead” to more projection than other aspects or numbers. Thus, the full potential of septiles are never fully or remotely realized in practice.
Think of what the spirit (3) means to you and you will then have a reasonably good idea of what a septile may symbolize.