[Ethereum] Why does the account balance show zero

When doing a sync with geth --fast, you will not get any state information until the sync is fully complete. A slow sync will give you incremental state along the way, but take significantly longer.

geth --fast has an interesting effect: geth cannot provide any information about accounts or contracts until the sync is fully complete.

Try querying the balance again after eth.syncing returns false.

Why?

The "fast" sync looks nearly complete because the reported current block is based on the best header you have. But that doesn't give you any information about how much of the account state data you have downloaded.

geth can sync the blocks much faster than it can download the state. An excerpt from @karalabe's explanation why:

This trie data structure is an intricate interlink of hundreds of millions of tiny cryptographic proofs (trie nodes). To truly have a synchronized node, you need to download all the account data, as well as all the tiny cryptographic proofs to verify that noone in the network is trying to cheat you. This itself is already a crazy number of data items. The part where it gets even messier is that this data is constantly morphing: at every block (15s), about 1000 nodes are deleted from this trie and about 2000 new ones are added. This means your node needs to synchronize a dataset that is changing 200 times per second. The worst part is that while you are synchronizing, the network is moving forward, and state that you begun to download might disappear while you're downloading, so your node needs to constantly follow the network while trying to gather all the recent data. But until you actually do gather all the data, your local node is not usable since it cannot cryptographically prove anything about any accounts.