百 caps 白 (the sound bái) with 一: a numeral built by borrowing a sound and stacking a stroke. Big round numbers came late to writing everywhere — Chinese solved it with elegant recycling.
百 is a hundred and, culturally, 'all of them': 百姓 ('the hundred surnames' — the common people), 百货 (department store: 'hundred goods'), 百分之百 (100% — total certainty). The classic child's primer 百家姓 lists the Hundred Family Names; 百度 (Baidu) took its name from a Song-dynasty line about searching for someone 'hundreds and thousands of times'. In Chinese, when you mean everyone and everything, you say a hundred.
One 一 stacked on white 白 — count to a HUNDRED.
百姓 'hundred surnames' = ordinary people; the real list (百家姓) actually contains over 500 surnames — rounding was generous.