It is a sample, not a census. The catalog is built by a crawler that grows outward from a seed list, so it is large and it is broad, but an account it has never reached cannot appear here however fast it is growing. Read the board as “the fastest risers among the accounts we track”, which is the claim it actually supports.
It measures followers, and followers are not reach. An account can add followers steadily while its videos are watched less than they were last month, and nothing on this page would show it. For reach you need per-post data, which is the thing TikTok does not give an outside reader.
It cannot tell you why an account is growing. A jump could be one video breaking out, a press mention, a cross-post from another platform, or bought followers. The board says what moved and by how much; the reason is a question for the account.