Sophie, that is exactly what happens with us. My dd is 18 months and having successfully got her to bed using wi/wo by 6.30, sometimes even earlier, she wakes almost every day at 5.30! She has an hour and a half or 2 hours sleep in the day so although it seems awful to be getting up so early, as someone pointed out to me recently, 11 hours at night an a good nap is all we can ask. My dd slept terribly before wi/wo, waking at night and still getting up early. So, like you said, I just go to bed earlier and try to appreciate that I have all of my evenings free from 6.30pm and actually, I always said that 6am was an acceptable start to the day and it's only 30 mins away from that. It's hard but it could be worse. I know, as we have had some horror nights in her 18 months!
Also, some days are different. Every now and then she wakes at 6 or 6.30 if she's maybe gone to bed a bit later (although that doesn't always work for us).
Sounds like your lo is sleeping really well. Well done for success with wi/wo. It's a good feeling so be proud to say she's sleeping well. We did it on henrysmamas suggestion and with her support we had bedtimes and night wakings cracked in 3 nights. This site has got me through some toughies!!