Here is a macOS display of signal strength (RSSI) and noise. Note RSSI and noise.

This is a fairly quiet area for Wi-Fi as shown by the noise floor at -93 dBm, and the access point signal is well above that floor at -54 dBm. The access points in this Wi-Fi network are configured for WPA2/WPA3 transitional, with this Mac using WPA3 pre-shared key.
If your noise is higher and/or is closer to your RSSI output power, you'll have communications issues and retries, and your bandwidth will drop, and connections can wobble.
Unfortunately for this case, iPhone and iPad aren't good at showing Wi-Fi details, which is why I pointed to using Mac or Windows to gather the info. Some Wi-Fi networks and some routers and access points can embed monitoring for this information and display it in the router or access point user interface, too.
As for trusting those "bars"... Wi-Fi and cellphone displays showing bars aren't all that meaningful, as those are as much intended and chosen to avoid folks making support calls as to represent the actual conditions, and the choices and thresholds can and do vary from version to version, within the products from a vendor, and vendor to vendor. They're akin to the gages and indicators found in modern vehicles; they're more for show than for diagnostics, very much the "check engine" or "chuck engine" light.