This was very common after 1996 when Lexmark sold their keyboard-making business to Unicomp. IBM still continued to produce their own Model Ms in the UK until 1999, including ones for the US market. This seemed to be limited to the standard 101-key, and the 122-keys - M13s, SSKs and other more specialized variants came from Unicomp (and Maxiswitch, for the M13s up till 1998 or so).
As to why IBM kept producing their keyboards instead of buying Unicomp ones, and how much of the parts were from Lexmark/Unicomp (I've seen 'blue label' Greenock-made Model Ms whose innards have Lexmark copyrights on them) remains a mystery.
Before 1996, there were US ANSI layout Model Ms from the Greenock plant which had their own part number. These were usually sold in EMEA markets which preferred the US layout (e.g. the Netherlands)
EDIT: To ruminate a little further on the topic: IBM had a contract with Lexmark until 1996 to produce keyboards, and the expiration of this contract is what led to their keyboard business being sold off. Reasons for continuing production in the UK likely involve one or both of the following reasons:
1) Unicomp's capacity for production was insufficient for IBM's needs (keep in mind that they did not retain the IBM/Lexmark factory, they moved to a smaller one in the same city).
2) IBM had external incentives for keeping the keyboard line in the UK running. For example, they continued manufacturing laptops and servers into the 2000s in the UK, long after it made any economical sense to do so, mainly because the local authorities paid them to keep the jobs open. What this meant was that the items were 80% assembled elsewhere, and then shipped to the UK for some final bits of work and inspection.
All the above aside, after the turn of the millennium, you will find examples of Unicomp-made IBM-branded 101-keys. This probably corresponded with IBM losing interest in the Model M, and contracting to Unicomp to provide spares.
