Oftentimes, the wording of what Christ or another character says will differ from Gospel to Gospel.
For example, here we have the angel speaking to the women after the resurrection, first in Matthew's account:
Matthew 28:5-7
5 And the angel answered and said unto the women, Fear not ye: for I know that ye seek Jesus, which was crucified.
6 He is not here: for he is risen, as he said. Come, see the place where the Lord lay.
7 And go quickly, and tell his disciples that he is risen from the dead; and, behold, he goeth before you into Galilee; there shall ye see him: lo, I have told you.
Now in Mark's account:
Mark 16:6-7
6 And he saith unto them, Be not affrighted: Ye seek Jesus of Nazareth, which was crucified: he is risen; he is not here: behold the place where they laid him.
7 But go your way, tell his disciples and Peter that he goeth before you into Galilee: there shall ye see him, as he said unto you.
The two passages say the same thing, but they are not the same at all.
There are many such examples like the above throughout the Gospels. What do we make of this?
Well, oftentimes the differences are as simple as omissions of words (or additions, depending on which account you're looking at). That means that one account's paraphrase of what was said leaves out or adds more detail, depending on the case.
Other times, a character is recorded as saying something different:
Mark 15:39
39 And when the centurion, which stood over against him, saw that he so cried out, and gave up the ghost, he said, Truly this man was the Son of God.
Luke 23:47
47 Now when the centurion saw what was done, he glorified God, saying, Certainly this was a righteous man.
Logically, these could be two separate statements that the man said, but also could be two paraphrases of the same statement.
Therefore, parallel accounts in the Gospels are different because an account may add or omit details, or statements may be paraphrased. If there is a paraphrase, however, it is guaranteed by God to be accurate and valid.
For example, almost every New Testament quotation from the Old Testament is a paraphrase.
Mark 1:2-3
2 As it is written in the prophets, Behold, I send my messenger before thy face, which shall prepare thy way before thee.
3 The voice of one crying in the wilderness, Prepare ye the way of the Lord, make his paths straight.
Mark 1:2 is a quotation from the book of Malachi:
Malachi 3:1
1 Behold, I will send my messenger, and he shall prepare the way before me: and the Lord, whom ye seek, shall suddenly come to his temple, even the messenger of the covenant, whom ye delight in: behold, he shall come, saith the Lord of hosts.
Mark 1:3 is from the book of Isaiah:
Isaiah 40:3
3 The voice of him that crieth in the wilderness, Prepare ye the way of the Lord, make straight in the desert a highway for our God.
Does Mark quote the verses verbatim? No, he paraphrases them.
Therefore it's important to recognize that if you see a slight difference in wording between the Gospels, it's because the wording of a paraphrase can differ considerably while retaining the same general message.
This also extends to events which overlap in the Old Testament. Oftentimes in the Old Testament the wording of the same event told in different accounts will differ. This is prevalent in Kings and Chronicles, for example, which often cover the same events.
So, what do we know as Christians? We know that what made it into the text of Scripture is a God-approved paraphrase of whatever was said. Therefore, we can rely on every word of for doctrine because it's guaranteed to be valid.
By comparing these paraphrases, it's typically very easy to get the intended message and meaning of what was said.
Oftentimes, the authors of the Bible will focus on different aspects of what was said, omitting certain things depending on their perspective of what was needed to report for that particular event to fit with the tone of the passage.
God guided this process and made sure that what we have is correct.