…All the livestock of the Egyptians died… [plague 5, Ex. 9:6]
…boils breaking out in sores on man and beast. [plague 6, Ex. 9:10]
The hail struck down everything that was in the field in all the land of Egypt, both man and beast….[plague 7, Ex. 9:25]
First off, Exodus 9:3 says that the fifth plague was on the livestock that were in the field: potentially, the livestock that wasn’t in the field was spared (this is certainly the case with the plague of hail [plague 7]: Ex. 9:20-21,25). This could mean that the animals that died were replaced by the animals that weren’t in the field when the plague struck, and it was this other set of animals that were then struck with boils.
It is also probable that Exodus is using the word “all” hyperbolically to describe ‘the totally devastating impact of the plague’: i.e., ‘So much of the livestock was killed, that in comparison those that survived were too few to be of significance’.1 As a note for Exodus 9:6 in the NET puts it:
The word “all” clearly does not mean “all” in the exclusive sense, because subsequent plagues involve cattle. The word must denote such a large number that whatever was left was insignificant for the economy….
Similar language is use to describe the destruction that the hail (plague seven) brought on the plants — ‘…the hail struck down every plant of the field and broke every tree of the field’ (Ex. 9:25) — and yet the author is aware that not every plant was destroyed: in the next chapter, Moses says a swam of locusts (plague eight) would come and ‘eat what is left to you after the hail, and they shall eat every tree of yours that grows in the field’ (Ex. 10:5; cf. v15). “All” is being used hyperbolically.
Additionally, ‘the Hebrew word kol, usually translated “all,” can mean “all sorts of” or “from all over” or “all over the place.” In this verse the better translation of the full expression would be “all sorts of Egyptian livestock died” or “Egyptian livestock died all over the place”’.2 Exodus 9:3 does seem to emphasize that “all sorts of” Egyptian livestock would be killed: ‘the horses, the donkeys, the camels, the herds, and the flocks’.
Another, less likely, suggestion is that the time between the fifth and sixth plagues and the sixth and seventh plagues was sufficiently long enough for the Egyptians to replenish their supply of livestock, only each time to then have them destroyed once more.
For more on figures of speech used in the Bible (of which hyperbole is one), see E. W. Bullinger’s classic work Figures of Speech Used in the Bible (1898) or Robert I. Bradshaw’s article Figures of Speech (1997).
1. John L. Mackay, Exodus (Fearn: Mentor, 2001), p. 166
2. Douglas K. Stuart, Exodus (Nashville: Broadman & Holman, 2006), p. 224