Good question. If we compare and contrast these 5 passages in these 2 letters of Paul, we see that:
Both Timothy and Titus were written to younger Church leaders dealing with false teaching
There is no single topic addressed in each of these sayings - they include affirmations of the Gospel, the worthiness of pursuing leadership, and that godliness is better than physical exercise
According to ESV study Bible, both letters were written mid-60s AD - so around the same time period
Most likely then, this is an expression Paul was using at this point in his life to emphasize points he wanted to make to young leaders, perhaps to emphasize points they should hold fast to in the midst of some specific question or concern.
Titus 3:4-7
But “when the kindness of God our Savior and his love for mankind
appeared, 5 he saved us not by works of righteousness that we have
done but on the basis of his mercy, through the washing of the new
birth and the renewing of the Holy Spirit, 6 whom he poured out on us
in full measure through Jesus Christ our Savior. 7 And so, since we
have been justified by his grace, we become heirs with the confident
1 Timothy 1:15
This saying is trustworthy and deserves full acceptance: “Christ Jesus
came into the world to save sinners”
1 Timothy 3:1
This saying is trustworthy: “If someone aspires to the office of
overseer, he desires a good work.”
1 Timothy 4:8-9
For “physical exercise has some value, but godliness is valuable in
every way. It holds promise for the present life and for the life to
come.” 9 This saying is trustworthy and deserves full acceptance.
2 Timothy 2:11-13
This saying is trustworthy: If we died with him, we will also live
with him. 12 If we endure, we will also reign with him. If we deny
him, he will also deny us. 13 If we are unfaithful, he remains
faithful, since he cannot deny himself.