I agree with you that their numbers are a "fudge". It used to be that the parish rector was required by canon law to purge the parish rolls every year or at least every three years. Communicants were also required to attend, at a minimum, three Sundays each year or they could be dropped from the parish rolls.
Like you, I removed myself from the Episcopal Church. A few years later, I learned that I had not been dropped from my former parish's roll. I knew of a number of others who were still being counted on the parish roll. This is quite common throughout TEC.
I'd wager that the Episcopal Church probably has about 1.2 to 1.5 million "baptized" members and probably less "Confirmed" members.
And, I read numbers as low as 600,000 who attend an Episcopal Church at least twice yearly (Christmas & Easter, of course). (From Barna Research, I think) I tend to believe the lower numbers are closer to the truth. There has been a rather steady exodus from the church for the past 40 years - that coupled with the aging membership & an apparant distate for God's command that they "be fruitful & multiply." Were they a species of animal, rather than a heretical religous sect, they would surely qualify for protection as a threatened or endangered species.