There are two ways to explain this:
1. Dispensationally.
2. Taking all of Scripture into consideration.
DISPENSATIONALLY
The period covered in the book of Acts is the transition period between the Fifth and Sixth dispensations.
Law is transitioning into Grace.
The revelation of the Holy Ghost in this period of time was progressive:
- At first, they got the Holy Ghost days after Jesus breathed on them.
- Then they got the Holy Ghost after an Apostle baptized them.
- They they got the Holy Ghost by the laying on of hands.
- Finally, they (we) got (get) the Holy Ghost at the point of salvation.
HERMENEUTICALLY
Scripture is additive.
That is, one shouldn't build a doctrine without taking all of Scripture into account.
My favorite example is the sign that was placed over Jesus' head at His crucifixion.
Matthew 27:37 And set up over his head his accusation written, THIS IS JESUS THE KING OF THE JEWS.
Mark 15:26 And the superscription of his accusation was written over, THE KING OF THE JEWS.
Luke 23:38 And a superscription also was written over him in letters of Greek, and Latin, and Hebrew, THIS IS THE KING OF THE JEWS.
John 19:19 And Pilate wrote a title, and put it on the cross. And the writing was, JESUS OF NAZARETH THE KING OF THE JEWS.
Putting it all together, the sign reads: THIS IS JESUS OF NAZARETH, THE KING OF THE JEWS.