Odds of a wild magic surge arent guarranteed. Its a 5% per lvl 1 (or higher) spell beeing cast. Infact, theyre even lower. Because the DM can then choose to let you roll on the wild magic table. He 'can choose' as in its not a guarranteed roll. And even if the roll happens its only a 5% chance of actually tirggering wild magic surge.
Some features can give yourself a bonus with the caveat that at any time between that, and taking a long rest the DM can then make you roll on the chart as well. Again, not a guarrantee that it will happen. Its up to the DM.
I dont know how they modelled it. My guess is just a flat 5% chance of it happening. But thats still a very low chance of it happening.
