Ignoring transpositions seems ok for a simple tool like the explorer. After all a french is just as good an opening reached from 1.d4, or 1e4.
For what's its worth when was playing actively OTB, found roughly 1 in 7 games tranposed into the french. Also the most common response was 1.d4 e6 2Nf3 c5 3c3. The motivation seemed to be wanted to play bf4/bg5 without having to recapture with knight after c5xd4. Players in English chess scene particularly keen to avoid main-line theory. Many 1.d4 players don't even risk playing c4 until see d5. If white decides to stay in 1.d4 lines 1...e6 has a few uses, i.e 2c4 Bb4ch, and 2Nf3 c5.
According to opening explorer after 1d4 e6, 2e4 occurs over 80,000 times, 2c4 about 16,000 times and 2Nf3 occurs about 6000 times...
I'm surprised that 1d4 players are prepared to transpose to playing against the French to the degree they apparently are ? (Or is this a pecularity of opening explorer that presupposes 1e4 has been played first ?)
Thoughts ?
Edit: I've just checked my own games and it is what I thought. There doesn't seem anyway of checking how many players who start with 1d4 are prepared to play against the French. Opening explorer just merges the moves...