This contradiction should have been enumerated during the book of Leviticus, particularly during the entries which dealt with Yahweh’s sacrifices on his appointed times (see #194-197, #198-204, and #205-208).
At any event, Numbers 15 is clearly an inserted piece of Priestly legislation which has neither any relationship to the spy story that precedes it nor the Korah rebellion (or the Dathan and Abiram rebellion) that follows it. It is a later piece of sacrificial legislation penned by the Aaronid priesthood that was inserted here and which specifies in a concise and orderly manner the specific measurements of each grain-offering that was to accompany the various different burnt-offerings.
So with regards to our particular contradiction, when a lamb was sacrificed to Yahweh as a burnt-offering, it was to be accompanied by a grain-offering
consisting of a one-tenth measure of semolina flour, mixed with one-fourth of a hin of oil, also wine for the libation in the amount of one-fourth of a hin. (Num 15:4-5)
This measurement for the lamb burnt-offering is attested elsewhere in the Torah, for example at Numbers 28 verses 13 & 29. Yet for what ever reason, the scribe of Leviticus 23:13 wrote down a double portion of semolina flour, two-tenths worth, for the grain-offering of the burnt-offering of 1 lamb.
Indeed, later rabbinical traditions tried to account for, or harmonize, these differences, but the fact remains that these are opposing measurements. Since the ritual legislation in the book of Numbers, both that of chapter 15 and 28, is considered to have been penned at a later date, and additionally since both Numbers 15 and 28 specify Yahweh’s sacrifices in a very detailed and precise manner, it might be inferred that these later passages were an attempt to codify what each accompanying grain-offering for each burnt-offering consisted of in a very precise manner. The double portion assigned by the scribe of Leviticus 23, therefore, might be representative of an earlier time when these cultic matters had not yet been codified. Additionally, each scribe presented his measurement, whether one-tenth or two-tenths, as the direct commandment of Yahweh. And once again, we see Yahweh being used as a mouthpiece for the contending views of the priestly scribes who wrote this legislation.