This is the single most misunderstood part of the whole system. Veterans call me furious: "I have a 50% and a 50% — where is my 100%?" The answer is buried in 38 CFR § 4.25, the combined ratings table.
The whole-person theory
The VA treats you as 100% healthy to start. Your first (highest) rating comes off that. The next rating is applied only to what is left. So a 50% rating leaves you "50% efficient," and the next 50% is taken from that remaining 50 — giving 75%, then rounded, not 100%.
What this means for you
Because of this math, small ratings stacked on top of large ones add very little. That is why we prioritize raising your biggest condition and pursuing TDIU when your service-connected conditions keep you from working — often a faster route to the 100% benefit level than chasing small add-ons.
Always check your combined rating against the table. Rounding errors and math mistakes happen, and they cost real money.