Siglent uses a Sigma-Delta ADC (believe ADI chip), while KS and Keithley utilize custom combinational multi-slope integrating ADC circuit with an external ADC chip (also believe an ADI chip). These more complex overall ADC methods enable KS and Keithley to "extend" resolution, the Siglent does not have such capability.
Siglent is good value, we have one, but not in same class as latest, more feature rich (and expensive) KS and Keithley DMMs like KS34465A and DMM6500 respectively (have these also).
For GP use the Siglent is fine, however for more involved use we lean towards the KS or Keithly.
Best,