Time for some facts...
For Emails, unless you look in the header information, the timestamp is meaningless. This is because it can be spoofed, i.e. deliberately set to any time in the valid range, including in the future. It can therefore be that a message is prepared for sending, marked at a given time, and not sent until a later time. Each mail server that handles a message will timestamp it, and this is reliable information, so if you look in the header you'll see the truth.
I would expect DIYnot to batch up Emails, not send them one at a time.
When in transit, there is no coordination between Emails in a given batch, so the one sent first can arrive before or after the one sent last. It could arrive several minutes before, or after. If a transit mail server crashed, or was restarted for any reason, then the message can be significantly delayed. Spam filtering also adds delays, and some filtering hosts are less efficient, or more loaded, than others.
Emails are delivered using a reliable, connection-oriented, Internet transport. This is why Emails don't just "go missing", like flaky people claim happens to them when they don't want to admit receiving a message. However, with the abundance of Spam and Spam filters, Emails are capable of never arriving. This is not because the transport is unreliable, because it isn't.
SMS texts are also stored-and-forwarded, and delivered by a connection-oriented transport, but the delivery is by an entirely different protocol set and medium.