Generally, CO had the low numbers for international routes while UA had the 800s and 900s. UA had low numbers for domestic routes. UA1/2 was ORD-HNL. Once the two carriers merged, they decided to mix and match...
For me at LAX, UA839 to SYD and UA923 to LHR are historic flight numbers that have not changed in at least the 15 years I have been flying United. LAX-MEL, however, is UA98.