this is btc from mybitcoin
It could be, but several of the people who have sent BTC to addresses for their mybitcoin accounts have noted that the funds have not been moved from those addresses.
How would they know this if we don't need to use the same address to send coins? Or are all transactions trackable because it's actually Address A (original receiving address) -> Address B (New sending adress), then Address B -> Address C (consolidating address) ?
That is correct. Every single fund split or move could be extracted from block chain, that's why use of consolidating address e.g. donation signatures creates fund movement loops and hurts everyone's privacy by making it easier to identify someone's spendings.
Prior to performing the analyses above, we expected the user network to be largely composed of disjoint trees representing Bitcoin flows between one-time public-keys that were not linked with other public-keys. However, our analyses reveal that the user network has considerable cyclic structure.
An Analysis of Anonymity in the Bitcoin System