DigitalOcean doesn't throttle the network speed of droplets. Your droplet, however, is sharing a hypervisor with a number of other droplets, depending on your size.
The reason you don't get this speed decrease when communicating between servers is that datacenters are peered very well. There is less latency than to your office connection. See the effect of bandwidth delay product on transfer speed here: http://en.wikipedia.org/wiki/Bandwidth-delay_product.
I suggest you open a ticket if you feel like the speed should be investigated. The support staff will take a look at the droplet and node for you.