Different speed of transactions confirmation

Why the transactions made later are confirmed quicker, than earlier transaction?

In my Zcash4win wallet I have 3 transactions. At 14:53 28 november I send zcash and after 6 hours this transation has 0 confirmations (I can see it at https://zcashnetwork.info as uncunfirmed). Later, at 15:49 and 20:48 28 november I received zcash and this transactions have 10 confirmations.