Why does unspent address index differ between APIs? I've been using Blocktrail's API:

https://www.blocktrail.com/api/docs/lang/nodejs#data_overview

to get data for the following unspent address on the testnet: 

2N6qMqR9D4BTkD6fS1h6Sq3kRV2FeB1L14j.

Blocktrail's API returns the index as 4, whereas blockexplorer's:

https://testnet.blockexplorer.com/api/addr/2N6qMqR9D4BTkD6fS1h6Sq3kRV2FeB1L14j/utxo

and smartbit's APIs:

https://testnet-api.smartbit.com.au/v1/blockchain/address/2N6qMqR9D4BTkD6fS1h6Sq3kRV2FeB1L14j/unspent

both return an index of 0. Am I missing something here?
both return an index of 0. 


0 is the correct answer here.


  Am I missing something here?


Not sure.

I can't reproduce your problem. I'm querying blocktrail like this:

curl https://api.blocktrail.com/v1/tbtc/address/2N6qMqR9D4BTkD6fS1h6Sq3kRV2FeB1L14j/transactions?api_key=MY_APIKEY


and that returns all of the transactions for the address, none of which have an index of 4.

Can you post the line of code that calls their API?