Why not use tx_hash as trace id?

[fishTsai20]

void McBlockEmulator::create_trace(const TransactionInfo& tx, td::Promise<Trace *> promise) {
Trace *trace = new Trace();
trace->emulated = false;
trace->workchain = tx.account.workchain;
trace->transaction_root = tx.root;
trace->id = tx.initial_msg_hash.value();
trace->node_id = tx.in_msg_hash;

Why use the in msg hash of the first tx as the id of the trace, instead of the tx hash of the first tx?
The same msg hash may belong to a different tx. How can I find his tx through this msg hash?
https://tonscan.org/tx/dda06042340501f32cb9cd0357167174067d5efd3e86a4f5c1b8cc753b6783b0
Such as tx: 84dRUUkH2DIfD98x0Lrt3cbVdG+2b/hiU9l4qdAZl+M= and tx: 00Mc+vAtxtsjzoXzFZV9gttbxEYwQf8+GlkamFjnL+Y= have same msg hash:wnbiJ48KLJva9Aal8+8GZN8PMxlKVxJx3Hw3fx6r+AI=