How expanded tables work

Your confusion lies in the fact that even though an expanded table is a series of left-outer joins of tables from many -> 1, the expanded table still has all the columns of all the tables.  And tables are a collection of independent columns.  Meaning that a filter on one column does not affect another until after applied to the model. (and even then the impact if from cross filtering rows)  Take this code here:  

Sum Amount = CALCULATE(SUM(t3Sales[Amount]),t3ProductName[ProductID])

internally, it is written as this:

Sum Amount =
CALCULATE (
    SUM ( t3Sales[Amount] ),
    FILTER ( ALL ( t3ProductName[ProductID] ), t3ProductName[ProductID] )
)

So it will return all the ProductID's from the t3ProductName column.  But the table also has a filter on t3Sales[ProductID] because you put it on the rows which when combined filters down the t3Sales table to just where t3Sales[ProductID] = 100 for row 1, and so on for rest of the table.  Grand total has no filter on t3Sales[ProductID] and only has the above filter on tsProductName[ProductID] column which returns all values, so you get the correct grand total.

next is your Sum Amount2

Sum Amount2 = CALCULATE(SUM(t3Sales[Amount]),t3Sales[ProductID])

which internally is rewritten as:

Sum Amount2 =
CALCULATE (
    SUM ( t3Sales[Amount] ),
    FILTER ( ALL ( t3Sales[ProductID] ), t3Sales[ProductID] )
)

So, even though you put t3Sales[ProductID] on the rows, since you also referenced that column as a filter argument to CALCULATE, the table row filter is overridden by the above FILTER statement which returns all ProductID for t3Sales for every row which is why you get the grand total amount for every row in table.  Difference from Sum Amount is now there is no filter on the t3ProductName[ProductID].  

Make sense?

"Context Transition" refers to moving a column filter from row contex to filter context.  

"Modify filter context" means just that.  When you have columns on the rows, columns, filter area, slicers of report that is the initial filter context.   Either row context transition or arguments to 'CALCULATE' may change ( or modify) that filter context.