Can't understand the difference between DatesTable[Date] vs DatesTable[Date].[Date]

Hello, this is my first post. And I have a lot to learn yet.

I'm puzzled with the difference between the items stated on the subject line. What does the dot notation do? I'm finding it hard to google answers on this.

Anyways, what caused me confusion was this scenario:

This measure seemed to work fine:

Shipments Prev Year = CALCULATE(sum(ShippingActivity[Shipments]), SAMEPERIODLASTYEAR(Dates[Date]))

It was further used for these two:

Shipment Incr = SUM(ShippingActivity[Shipments])-ShippingActivity[Shipments Prev Year]

Shipment % Incr = divide(Shipment Incr , ShippingActivity[Shipments Prev Year])

I made a bar chart with the Shipment % Incr, using Year as an axis. All good.

When I tried to use the Warehouse column as the Legend, I got an error saying the graph couldn't be displayed.

Finally I was able to make the Warehouse as Legend to work when I changed the Shipments Prev Year formula to this:

CALCULATE(sum(ShippingActivity[Shipments]), SAMEPERIODLASTYEAR(Dates[Date].[Date]))

Why did it work this way?

Thanks,

Ed